Our attitude toward sin, a true sorrow for sin. (Wiersbe)

Blessed is the man who mourns like one mourning for the dead. (Barclay)

Happy the mourning (Young's Literal)

Blessed the mourning ones.

(Notice that the verb "are" is not present in the original Greek and has been added by most of the translations in this beatitude as well as all the others).

Blessed (see makarios) means spiritual prosperous, independent of circumstances because it is a state bestowed by God and not a feeling felt.

Note that Jesus does not mean, "Blessed are grim, cheerless Christians" and neither does He mean, "Blessed are those who are mourning over the difficulties of life" as is explained in more detail below.

Adam Clarke writes that...

the person whom Christ terms happy is one who is not under the influence of fate or chance, but is governed by an all-wise providence, having every step directed to the attainment of immortal glory, being transformed by the power into the likeness of the ever-blessed God. (Clarke, A. Clarke's Commentary: Matthew)

The Amplified Version (which often functions much like a "mini-lexicon") adds

Blessed and enviably happy [with a happiness produced by the experience of God’s favor and especially conditioned by the revelation of His matchless grace]

“Lord, let me weep for nought but sin,
And after none but thee;
And then I would-oh, that I might-
A constant mourner be!”

Mourn (3996) (pentheo from pénthos = mourning) means to mourn for, lament. Mourning is grief and sorrow caused by profound loss, and we usually associate it with death.

Mourning can reflect an outward expression of sorrow. It is to experience sadness or grief as the result of depressing circumstances or the condition of persons and so to be sad, to grieve, to bewail or to lament. 

Sinclair Ferguson asks...

Is Jesus, then, giving us a word of general encouragement in what he says here, assuring us that sorrow will eventually abate? Is he saying, 'Keep going. It will soon pass. Time heals all wounds'? That would be far too superficial a reading of the Sermon on the Mount. Jesus is speaking about life in the kingdom of God. The poverty he describes is in a man's spirit, not his pocket. Similarly, the grief Jesus describes is man's mourning over his own sinfulness; it is regret that he has proved a disappointment to the Lord. Numbed by the discovery of his poverty of spirit, he learns to grieve because of it. Here, then, is another characteristic of the Christian. He does not excuse his sin, or belittle it, or ignore it...As with all spiritual graces, it is possible for us to be deceived about the real nature of this mourning. It is emphatically not to be equated with a heavy and depressive spirit. It is emphatically not to be equated with a heavy and depressive spirit. Some of us by nature are melancholic, and sink more easily in our spirits. We become introverted and develop a poor image of ourselves that surfaces in the way we look at or address others, even in the way we hold our heads and walk. But all of these things can be characteristics of a person who is absorbed in himself; rather than is poor in spirit. By contrast, the man who genuinely mourns because of his sin has been drawn out of himself to see God in his holiness and grace. It is this – his sight of God – that has made him mourn. Paradoxically, it is the same sight of God that will bring him comfort. The God against whom he has sinned is one who forgives sinners! (Ferguson, Sinclair: Sermon on the Mount :Banner of Truth)

uses pentheo 10 times: Matthew 2x; Mark; Luke; 1 Corinthians; 2 Corinthians; James; Revelation 3x. It is translated mourn, 6; mourned, 1; mourning, 3. The KJV translates it  bewail, 1; mourn, 7; wail, 2.

Matthew 5:4 "Blessed are those who mourn, for they shall be comforted.

Matthew 9:15 And Jesus said to them, "The attendants of the bridegroom cannot mourn as long as the bridegroom is with them, can they? But the days will come when the bridegroom is taken away from them, and then they will fast.

Mark 16:10 She went and reported to those who had been with Him, while they were mourning and weeping.

Luke 6:25  "Woe to you who are well-fed now, for you shall be hungry. Woe to you who laugh now, for you shall mourn and weep.

1 Corinthians 5:2 And you have become arrogant, and have not mourned instead, in order that the one who had done this deed might be removed from your midst.

2 Corinthians 12:21 I am afraid that when I come again my God may humiliate me before you, and I may mourn over many of those who have sinned in the past and not repented of the impurity, immorality and sensuality which they have practiced.

James 4:9 Be miserable and mourn and weep; let your laughter be turned into mourning, and your joy to gloom.

Revelation 18:11 (note)  "And the merchants of the earth weep and mourn over her, because no one buys their cargoes any more;

Revelation 18:15 (note) "The merchants of these things, who became rich from her, will stand at a distance because of the fear of her torment, weeping and mourning,

Revelation 18:19 (note) "And they threw dust on their heads and were crying out, weeping and mourning, saying, 'Woe, woe, the great city, in which all who had ships at sea became rich by her wealth, for in one hour she has been laid waste!'

 

Pentheo is found 45 times in the Septuagint or LXX. the first use describing the mourning of Abraham for his wife Sarah (Ge 23:2). Pentheo describes Jacob mourning for his son Joseph whom he thought had been killed (Ge 37:34-35). Samuel grieving over Saul and his failure to obey (1Sa 15:35, 16:1).  David for his son Absalom (2Sa 13:37, 19:1). All Judah and Jerusalem for King Josiah (2Chr 35:24). Ezra mourning over the unfaithfulness of the exiles in marrying foreign women. Ezra 10:6, cf 10:1-2). Nehemiah mourning over the great distress of the remnant who were back in Jerusalem (Neh 1:4). The people weeping and mourning upon hearing the Words of the Law read (Neh 8:9). Figuratively of Zion or Jerusalem's gates mourning over the coming destruction (Isa 3:6). When Messiah returns to comfort all who mourn (Isa 61:2-3). Daniel mourned for 3 entire weeks (Da 10:1) Several verses refer to "the land" (the land of Israel) mourning over the sin of the chosen people (cf Jer 23:10 because of the curse, Hos 4:3, Joel 1:10)

It is interesting to note that the Greek Stoics regarded such mourning as something to be avoided and the pointlessness (as seen in this secular view) was a popular theme in Greek philosophy. One imagines what their shock must have been to read Jesus' words which can be paraphrased as

"Happy those who continually mourn as one laments over a loved one who had died"!

Trench says that pentheo means

to grieve with a grief which so takes possession of the whole being that it cannot be hid. (Trench, R. C. Synonyms of the New Testament)

Pentheo is the word which is used for mourning for the dead, for the passionate lament for one who was loved. This is the quality of mourning Jesus is calling for as we see our sins the way God sees them and His Spirit convicts us of sin. (cf John 16:8, cf Acts 2:37, Zechariah 12:10)

The English word mourn means to feel or express grief or sorrow.

In Classical Greek in most uses of pentheo, it expresses a sorrow which is outwardly expressed in some way, such as by tear or laments. Among the Greeks the verb and noun (penthos) were used especially for public mourning.

Not surprisingly, pentheo is often connected with the term "weep" and it describes the mourning which cannot be hidden. It describes not only a grief which brings an ache to the heart, but also a grief which brings tears to the eyes.

Barclay comments that pentheo is...

It is defined as the kind of grief which takes such a hold on a man that it cannot be hid. It is not only the sorrow which brings an ache to the heart; it is the sorrow which brings an ache to the heart; it is the sorrow which brings the unrestrainable tears to the eyes. Here then indeed is an amazing kind of bliss: Blessed is the man who mourns like one mourning for the dead. (Barclay, W: The Daily Study Bible Series, Rev. ed. Philadelphia: The Westminster Press)

Notice that here in Mt 5:4 Jesus uses pentheo in the present tense which speaks of a continual state of mourning or mourning as one's lifestyle! Spiritual mourning is not just an isolated or limited act in life. It is a continual part of the believer’s life. The present tense shows that spiritual mourning has lasting dimensions in the life of the Christian. Wuest conveys the sense of the verb tense rendering it...

Spiritually prosperous are those who are mourning, because they themselves shall be encouraged and strengthened by consolation.

MacArthur notes that in Greek there are nine words that express sorrow, but that...

of the nine terms used for sorrow, the one used here (pentheo, mourn) is the strongest, the most severe. It represents the deepest, most heart-felt grief, and was generally reserved for grieving over the death of a loved one. It is used in the Septuagint (Greek Old Testament) for Jacob’s grief when he thought his son Joseph was killed by a wild animal (Gen. 37:34). It is used of the disciples’ mourning for Jesus before they knew He was raised from the dead (Mark 16:10). It is used of the mourning of world business leaders over the death of its commerce because of the destruction of the world system during the Tribulation (Rev. 18:11, 15).

The word carries the idea of deep inner agony, which may or may not be expressed by outward weeping, wailing, or lament. When David stopped hiding his sin and began mourning over it and confessing it (Ps. 32:3–5), he could declare, “How blessed is he whose transgression is forgiven, whose sin is covered! How blessed is the man to whom the Lord does not impute iniquity, and in whose spirit there is no deceit!” (vv. 1–2). (MacArthur, J: Matthew 1-7 Chicago: Moody Press)

As A W Pink so eloquently phrases it (see full quote below)...

this “mourning” is by no means to be confined unto the initial experience of conviction and contrition, for observe the tense of the verb: it is not “have mourned,” but “mourn”—a present and continuous experience. The Christian himself has much to mourn over. The sins which he now commits—both of omission and commission—are a sense of daily grief to him, or should be, and will be, if his conscience is kept tender. An ever-deepening discovery of the depravity of his nature, the plague of his heart, the sea of corruption within—ever polluting all that he does—deeply exercises him. Consciousness of the surgings of unbelief, the swellings of pride, the coldness of his love, and his paucity of fruit, make him cry, “O wretched man that I am.” (Ro 7:24) (Matthew 5:3-4: The Beatitudes) (Bolding added)

Pentheo "is most frequent in the LXX for mourning for the dead, and for the sorrows and sins of others" (McNeile).

"There can be no comfort where there is no grief" (Bruce).

Sorrow should make us look for the heart and hand of God and so find the comfort latent in the grief. (But remember that in the present context the grief is not sorrow in general as affects all mankind but sorrow over grieving the heart of God with our sins against Him, cf Gen 39:9, 2Sa 12:13, Ps 51:3-4, Ezek 6:9).

“Lord, let me weep for nought but sin,
And after none but thee;
And then I would-oh, that I might-
A constant mourner be!”
(C H Spurgeon)

This word describes brokenness over our estrangement by our sin and how prone we are to wander.

Ps 34:18 The Lord is near to the brokenhearted, and saves those who are crushed in spirit.

Spurgeon's comment: The Lord is nigh unto them that are of a broken heart. Near in friendship to accept and console. Broken hearts think God far away, when He is really most near them; their eyes are holden so that they see not their best Friend. Indeed, He is with them, and in them, but they know it not. They run hither and thither, seeking peace in their own works, or in experiences, or in proposals and resolutions, whereas the Lord is nigh them, and the simple act of faith will reveal Him.

And saveth such as be of a contrite spirit. What a blessed token for good is a repentant, mourning heart! Just when the sinner condemns himself, the Lord graciously absolves him. If we chasten our own spirits the Lord will spare us. He never breaks with the rod of judgment those who are already sore with the rod of conviction. Salvation is linked with contrition.

Ps 51:17 The sacrifices of God are a broken spirit; A broken and a contrite heart, O God, Thou wilt not despise.

Spurgeon's Comment:  The sacrifices of God are a broken spirit. All sacrifices are presented to thee in one, by the man whose broken heart presents the Saviour's merit to thee. When the heart mourns for sin, thou art better pleased than when the bullock bleeds beneath the axe. "A broken heart" is an expression implying deep sorrow, embittering the very life; it carries in it the idea of all but killing anguish in that region which is so vital as to be the very source of life. So excellent is a spirit humbled and mourning for sin, that it is not only a sacrifice, but it has a plurality of excellences, and is preeminently God's sacrifices. A broken and a contrite heart, O God, thou wilt not despise. A heart crushed is a fragrant heart. Men contemn those who are contemptible in their own eyes, but the Lord seeth not as man seeth. He despises what men esteem, and values that which they despise. Never yet has God spurned a lowly, weeping penitent, and never will he while God is love, and while Jesus is called the man who receiveth sinners. Bullocks and rams he desires not, but contrite hearts he seeks after; yea, but one of them is better to him than all the varied offerings of the old Jewish sanctuary.

Isa 57:15 For thus says the high and exalted One Who lives forever, whose name is Holy, "I dwell on a high and holy place, and also with the contrite and lowly of spirit in order to revive the spirit of the lowly and to revive the heart of the contrite.

Spurgeon comments: That is a wonderful verse. You notice that the prelude to it explains the greatness and the holiness of God; and then, like an eagle swooping out of the shy even down to the earth, we find God coming from his high and lofty place to dwell with humble and contrite hearts. Not with the proud,— not with you who think yourselves good and excellent,— does God dwell; but with men who feel their sin, and own it; with men who feel their unworthiness, and confess it. I will read this verse again to impress it upon your memory: “Thus saith the high and lofty One that inhabiteth eternity, whose name is Holy; I dwell in the high and holy place, with him also that is of a contrite and humble spirit, to revive the spirit of the humble, and to revive the heart of the contrite ones.”

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F B Meyer (Our Daily Walk) comments: THIS VERSE has reference to God's two Homes--the macrocosm of the great universe and the microcosm of the human heart. Our God is so great that the Heaven of heavens cannot contain Him, but He is so lowly and humble that He will stoop to fill the heart of a child. He bids us learn of Him, for He is meek and lowly in heart.

The humble and contrite heart. It seems almost too wonderful to believe that the Eternal One will care to come and live with the child of Time; that the Infinite and Holy God will descend to the narrow limits of a human heart! (see John 14:23).

PRAYER
Spirit of purity and grace,
Our weakness, pitying, see;
O make our hearts Thy dwelling-place,
And worthier Thee. AMEN.

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J. C. Philpot (Daily Portions) comments: O what a mystery that God should have two dwelling-places! The "heaven of heavens" that "cannot contain him;" and the humble, broken, and contrite heart! But in order that the Lord of heaven might have a place in which he could live and lodge, God gives to his people gifts and graces; for he cannot come and dwell in the carnal mind, in our rebellious nature, in a heart full of enmity and wickedness; he therefore makes a lodging-place for himself, a pavilion in which the King of glory dwells, the curtains of which are like the curtains of Solomon. His abode is that holy, divine nature which is communicated at regeneration--"the new man, which after God is created in righteousness and true holiness." Thus Christ dwells in the heart by faith; and is "in his people, the hope of glory." And this made Paul say, "I am crucified with Christ; nevertheless I live; yet not I, but Christ lives in me; and the life which I now live in the flesh, I live by the faith of the Son of God, who loved me, and gave himself for me."

This is the object of God's dealings--that the Lord God might dwell in his people; that there might be a union between the Church and her covenant Head--"I in them, and they in me, that they might be perfect in one." This is the unfolding of the grand enigma, the solution of the incomprehensible mystery, "God manifest in the flesh,"--that the Lord God might dwell in his people; "I will dwell in them, and walk in them; and I will be their God, and they shall be my people;" and thus glorify himself by filling their hearts with his grace and glory, as Solomon's temple was of old, and that they might enjoy him, and be with him when time shall be no more. This is the grand key to all the Lord's dealings with the soul, and all his mysterious leadings in providence--that the Lord God might dwell in the hearts of his people here, and be eternally glorified in them in a brighter and a better world.

Isa 66:1,2 Thus says the LORD "Heaven is My throne, and the earth is My footstool. Where then is a house you could build for Me? And where is a place that I may rest? For My hand made all these things. Thus all these things came into being," declares the Lord. "But to this one I will look. To him who is humble and contrite of spirit, and who trembles at My word.

Spurgeon comments: The text first of all teaches us that God rejects all material temples as the places of his abode; but, secondly, it informs us that God has made a choice of spiritual temples, wherein he will dwell...

Only God is pleased to say that the man who trembleth at his word, the man of broken heart, the man who is poor in spirit, is such an one as he will look to; these are his temples, — these, and these only, the men in whom he will dwell.  And I am so thankful for this, beloved friends, because this is a state which, through God’s grace, is attainable by all here whom the Lord shall call...

 ...“of a contrite spirit,” that is, the man that feels his sin and hates it, that mourns that he should have rebelled against God, and desires to find mercy. Now, God will come to such, because there is purity in that heart. “Oh,” saith the contrite spirit, “I do not see any purity in my heart.” No, but what do you see, then? “Oh, I see all manner of sin and evil, and I hate myself because it is so.” There is purity in that hatred; at any rate there is a something that God loves in that hatred in your soul, of the sin that is within, and He will come to you, for there is something there that is akin to His own holiness: He has put it there. You have begun to appeal for mercy. Oh, then, God’s mercy will come, for mercy delights to visit misery. Mercy is always at home where there is a sinner confessing sin...

...I will close, lastly, with this: Those that are of this character secure A Great Blessing. God says he will look to them. That means several things. It means consideration. Whoever and whatever God may overlook, he will look upon a broken heart. This means approbation. Though God does not approve of the most costly building that is meant to be his house, he approves of every one that trembles at his word. It means acceptance. Though God will accept no materialism in his worship, he will accept the sighs and cries of a poor broken spirit. It means affection. Be they who they may that do not receive God’s help, contrite spirits shall have it. And it means benediction. “To this man will I look.” I was reading the other day in an old author the following reflection as near as I can remember it. Saith he, “There may be a child in the family that is very weak and sickly. There are several others that are also out of health, but this one is sorely ill. And the mother says to the nurse, ’You shall see after the rest, but to this one will I look — even to this one that is so sore sick and so exceeding weak.’” So God does not say to his angels, “You shall look after the poor and the contrite, I have other things to do,” but he saith, “Go ye about, ye spirits, be ye ministering spirits to those that are stronger, and bear them up in your hands, lest they dash themselves against a stone; but here is a poor soul that is very poor: I will look after him myself. Here is a poor spirit that is very broken: I will bind that up myself. Here is a heart that trembles very much at my word: I will comfort that heart myself;” and so, he that telleth the number of the stars, and calleth them all by name — he healeth the broken in heart; he bindeth up their wounds. Out of special love to them he will do it himself. I should like to be the means of comfort to some contrite spirit to-night. Very likely the Lord will say, “No, I will not make you the means of it.” Very well, Master: be it as thou wilt; but thou wilt do it thyself. When we write books and tracts, we wish that we might comfort the desponding. Very likely the Lord will say, “No, no.” What should we reply to this? “Lord, thou canst do it better than we could. There are some sores we cannot reach, some diseases that laugh at our medicines, but, good Lord, thou canst do it.” And the Lord will come to you, poor broken down in heart, — he will come. Don’t despair. Though the devil says you will never be saved, don’t believe it; and above all, turn your eyes full tears to Christ on the cross, and trust him. There is salvation in no other, but there is salvation in the crucified Redeemer. (Read Spurgeon's full message on this passage - Living Temples for the Living God)

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Many despise warning, and perish. Happy is he who trembles at the word of the Lord. Josiah did so, and he was spared the sight of the evil which the Lord determined to send upon Judah because of her great sins. Have you this tenderness? Do you practice this self-humiliation? Then you also shall be spared in the evil day. God sets a mark upon the men that sigh and cry because of the sin of the times. The destroying angel is commanded to keep his sword in its sheath till the elect of God are sheltered: these are best known by their godly fear, and their trembling at the Word of the Lord. (Faith's Checkbook - see April 3)

Isaiah 61:3 To grant those who mourn in Zion, Giving them a garland instead of ashes, The oil of gladness instead of mourning, The mantle of praise instead of a spirit of fainting. So they will be called oaks of righteousness, The planting of the LORD, that He may be glorified. (C H Spurgeon has an entire book on this one verse! "The Mourner's Comforter") (See also Spurgeon's sermon Comfort and Comforting or listen to the MP3)

The great characteristic of Jeremiah, the Weeping Prophet, was that he wept for his people (Jeremiah 9:1; 13:17).

David Brainerd's journal on October 18, 1740 has this entry...

"In my morning devotions my soul was exceedingly melted, and bitterly mourned over my exceeding sinfulness and vileness."

John MacArthur has an excellent discussion of "How can I truly mourn over sin?" The following discussion summarizes his thoughts. On the "negative side" you need to remove the things which hinder you from mourning, especially "the things that make us content with ourselves, that make us resist God’s Spirit and question His Word, and that harden our hearts. A stony heart does not mourn. It is insensitive to God".  One of greatest hindrances is a failure to let go of our love for a pet sin. Be assured that this will turn your heart to stone. Puritan Thomas Watson writes that the love of sin “makes sin taste sweet and this sweetness in sin bewitches the heart”.  (cf note Hebrews 3:13)

MacArthur also mentions other hindrances including despair, conceit, presumption, procrastination ("one of these days I'll take a look at my sins"...sure you will! Not! Delays do not make Christianity easier. The folly of taking your time when you stand under divine judgment makes less sense than purposely sleeping in a house that you know is on fire), and excessive merriment (click Happy are the Sad and scroll down).

On the positive side we can cultivate a heart soil that is fertile for the growth of genuine mourning over sins (and those in our church, our community, our country) by getting a fresh glimpse of the holiness of God, especially as demonstrated in His sacrifice for sins on the Cross. (e.g., see notes 1 Peter 1:14, 15-16, 1:17, 18-19). This vital discipline has been beautifully expressed by Christina Rossetti in her poem "Good Friday"

Am I a stone and not a sheep,

That I can stand, O Christ, beneath Thy cross,
To number drop by drop Thy Blood’s slow loss

And yet not weep?

Not so those women loved

Who with exceeding grief lamented Thee;
Not so fallen Peter weeping bitterly;

Not so the thief was moved;

Not so the Sun and Moon

Which hid their faces in a starless sky.
A horror of great darkness at broad noon-

I, only I.

Yet give not oe’r

But seek Thy sheep, true Shepherd of the flock;
Greater than Moses, turn and look once more

And smite a rock.

One has to be careful that their mourning is not just an emotional reaction but that there is true confession, repentance and genuine mourning. John MacArthur addresses this question of how one can know they are mourning as Christ teaches...

Knowing whether or not we have godly mourning is not difficult. First, we need to ask ourselves if we are sensitive to sin. If we laugh at it, take it lightly, or enjoy it, we can be sure we are not mourning over it and are outside the sphere of God’s blessing. (see for example Saul's regret not mourning over his sin in 1Sa 15:30)...The godly mourner will have true sorrow for his sins. His first concern is for the harm his sin does to God’s glory, not the harm its exposure might bring to his own reputation or welfare. If our mourning is godly we will grieve for the sins of fellow believers and for the sins of the world. We will cry with the psalmist, “My eyes shed streams of water, because they do not keep Thy law” (Ps. 119:136). We will wish with Jeremiah that our heads were fountains of water that we could have enough tears for weeping (Jer. 9:1; cf. Lam. 1:16). With Ezekiel we will search out faithful believers “who sigh and groan over all the abominations which are being committed” around us (Ezek. 9:4; cf. Ps. 69:9). We will look out over the community where we live and weep, as Jesus looked out over Jerusalem and wept (Luke 19:41). The second way to determine if we have genuine mourning over sin is to check our sense of God’s forgiveness. Have we experienced the release and freedom of knowing our sins are forgiven? Do we have His peace and joy in our life? Can we point to true happiness He has given in response to our mourning? Do we have the divine comfort He promises to those who have forgiven, cleansed, and purified lives? The godly mourners “who sow in tears shall reap with joyful shouting. He who goes to and fro weeping, carrying his bag of seed, shall indeed come again with a shout of joy, bringing his sheaves with him” (Ps 126:5–6)." (MacArthur, J: Matthew 1-7 Chicago: Moody Press)

William Barclay commenting on "blessed are those who mourn" reminds us that...

the very first word of the message of Jesus was, “Repent!” No man can repent unless he is sorry for his sins. The thing which really changes men is when they suddenly come up against something which opens their eyes to what sin is and to what sin does. A boy or a girl may go his or her own way, and may never think of effects and consequences; and then some day something happens and that boy or girl sees the stricken look in a father’ or a mother’s eye’s; and suddenly sin is seen for what it is....Christianity begins with a sense of sin. Blessed is the man who is intensely sorry for his sin, the man who is heart-broken for what his sin has done to God and to Jesus Christ, the man who sees the Cross and who is appalled by the havoc wrought by sin. It is the man who has that experience who will indeed be comforted; for the experience is what we call penitence, and the broken and the contrite heart God will never despise (Psalm 51:17). The way to the joy of forgiveness is through the desperate sorrow of the broken heart. (Barclay, William: New Testament Words:. Westminster John Know Press, 1964)

Richards writes concerning "blessed are those who mourn"...

It is best to understand this phrase in the context of Jesus' purpose in the Beatitudes, where he contrasted the values of his kingdom with those of the world. This world considers blessed, not those who mourn, but the hedonistic and pleasure-seeking, who find "happiness" in transitory experience. It is the one who is dissatisfied and pained by what this world has to offer who will find the comfort that is offered by a living relationship with God. (Richards, L O: Expository Dictionary of Bible Words: Regency)

Chambers alludes to "those who mourn" over sin and relates it to repentance writing that...

J C Ryle writes that "those who mourn" refers to

those who sorrow for sin, and grieve daily over their own shortcomings. These people are more concerned about sin than about anything on earth: the remembrance of it is grievous to them; the burden of it is intolerable. Blessed are all such! “The sacrifices of God are a broken spirit” and a contrite heart (Psalm 51:17). One day they will weep no more: “they will be comforted.” (Ryle, J. C. Matthew)

Adam Clarke comments that...

those who, feeling their spiritual poverty (Mt 5:3), mourn after God, lamenting the iniquity that separated them from the fountain of blessedness. Every one flies from sorrow, and seeks after joy, and yet true joy must necessarily be the fruit of sorrow. The whole need not (do not feel the need of) the physician, but they that are sick do; i.e. they who are sensible of their disease (Mt 9:10-13). Only such persons as are deeply convinced of the sinfulness of sin, feel the plague of their own heart, and turn with disgust from all worldly consolations, because of their insufficiency to render them happy, have God’s promise of solid comfort.  (Clarke, A. Clarke's Commentary: Matthew) (Bolding added)

Spurgeon...

These seem worse off than the merely poor in spirit, for "they mourn." They are a stage higher, though they seem to be a stage lower. The way to rise in the kingdom is to sink in ourselves. These men are grieved by sin, and tried by the evils of the times; but for them a future of rest and rejoicing is provided. Those who laugh shall lament, but those who sorrow shall sing. How great a blessing is sorrow, since it gives room for the Lord to administer comfort! Our griefs are blessed, for they are our points of contact with the divine Comforter. The beatitude reads like a paradox, but it is true, as some of us know full well. Our mourning hours have brought us more comfort than our days of mirth. (The Gospel of the Kingdom: A Popular Exposition of the Gospel According to Matthew)

Guzik writes that...

The ancient Greek grammar indicates an intense degree of mourning. Jesus does not speak of casual sorrow for the consequences of our sin, but a deep grief before God over our fallen state. What do those who mourn actually mourn about? Their mourning is over just anything, but they mourn over sin. To really be followers of Jesus, we must mourn over our sin and the ruin and separation from God that comes to our life from sin.  We also mourn the general destruction and separation sin brings, far beyond the personal consequences to ourselves.

Dwight Pentecost quips that...

Our Lord did not promise, “Blessed are they that moan, for they shall. be comforted,” but, “Blessed are they that mourn.” When we carry some burden that brings tears, our natural response is to complain, to moan, to question God’s wisdom and benevolence, God’s right to do this to us. He did not say, “Those who moan will be comforted,” but, “those who mourn.” The biblical concept of mourning is recognizing a need, and then presenting that need to the God of all comfort. When one, in desperation, oppression, loneliness, bereavement, discouragement, anxiety, earnestness, desire, devotion, presents his need to God, God commissions the angels of heaven to dry tears from his eyes." (Pentecost, J. D. Design for living: Lessons in Holiness from the Sermon on the Mount. Kregel Publications)

Phil Newton draws an important distinction regarding the correct interpretation and application of Mt 5:4, noting that this...

is a favorite verse at funerals...as the assurance that in the time of bereavement and loss, they can be certain that God will give comfort. But this verse is not speaking to that issue. Others suppose that it is the assurance of comfort for some deprivation in life or some loss of perceived privilege. Some mourn because of being caught in a sin or deed that brings about certain consequences of great discomfort. So they comfort themselves that as they mourn they will eventually be comforted from this time of distress. They mourn over the penalty not over the deed. As the Puritan pastor in London, Thomas Watson, penned, “To mourn only for fear of hell is like a thief that weeps for the penalty rather than the offense” [The Beatitudes, 62]. There is no promise of comfort in this situation. Some mourn due to hurt feelings or perceived wrongs or personal injustices or the inability to accomplish personal goals, but that is not the type of mourning spoken of in this text. Mourning also has nothing to do with the habits of our lives. Some people are naturally melancholy so that they can easily weep or easily feel pity over a situation or quickly shed tears for a loss. But this is not a promise of blessing for a particular type of personality. To sum it up in one phrase, “It is not the sorrow of bereavement to which Christ refers, but the sorrow of repentance” [John Stott, 40-41].

...spiritual mourning does not begin by pointing at everyone else’s sins and shortcomings. It starts with me....How does this deep, inward spiritual mourning develop and continue in our lives?

1. It results from seeing God as holy. Sinclair Ferguson concurs, “It is this—his sight of God—that has made him mourn. Paradoxically, it is the same sight of God that will bring him comfort” [The Sermon on the Mount: Kingdom Life in a Fallen World, 19]. Where do we see God? We look into the pages of God’s Word, that infallible revelation of God. We meditate upon Scripture. We contemplate the Lord; see how he has worked in creation and most of all, in redemption. We look at the Incarnation of Jesus Christ, to his perfectly holy life, and then to the cross. Gaze upon Him who is so utterly holy that for Him to forgive just one sinner, it required that He pour out the vials of His wrath upon His own Son in a propitiatory fashion.

2. It is the apprehension of the nature of sin. David’s penitential Psalm 51 expressed this so clearly: “‘for I know my transgressions, and my sin is ever before me. Against You, You only, I have sinned and done what is evil in Your sight, so that You are justified when you speak and blameless when You judge” (Ps 51:3-4). Sin is ultimately an offense against God and his holy law. Thomas Watson adds, “David, that he might be a mourner, kept his eye full upon sin. See what sin is, and then tell me if there will be not enough in it to draw forth tears” [87]. It is God who is offended by our sin! It is God the Creator who lovingly sustains us, who even restrains us from following headlong after our own lusts, who tenderly places roadblocks in our way lest we face more of his judgment—it is this God against whom we sin!

3. It is the comprehension of sin as the source of our enmity with God, and consequently our hopelessness to change our own hearts. That produces spiritual mourning as we see the heavy toll requited for our sin: enemies of God; dead in trespasses and sins; children of wrath.

4. Spiritual mourning focuses upon sins in particular and not just general. It is not terribly difficult for a person to join the crowd by saying, “I’m a sinner,” for he generally adds, “I’m a sinner like everyone else.” But to get more specific, to address our own deeds, our thoughts, our attitudes, our tongue, our mistreatment of others, our neglect of spiritual disciplines, our ingratitude, our lusts, our impure thoughts, our disobedience to parents, our greed, our self-centeredness, our pride, and our arrogance, will bring us to the shocking reality of our sinfulness. Watson wrote, “A wicked man will say he is a sinner, but a child of God says, I have done this evil” [64].

5. Spiritual mourning produces hatred for sin and a repentant heart that desires to be holy. The American myth of Christianity without holiness knows nothing of spiritual mourning. When the sins of the world are just as prevalent within the church as outside the church, it tells us that true spiritual mourning—this character of true conversion—has not been near as widespread as statistics would state. Spiritual mourning targets sin, applies the cross of Christ, pleads with the Lord for deliverance, and exercises the spiritual disciplines that will help to shape the believer in conduct and character like that of Jesus Christ. He takes seriously Paul’s exhortation to the Ephesians, “Therefore be imitators of God, as beloved children” (Eph 5:1).

Do you know something of this spiritual mourning as a reality in your life? My friend, without it there will be no repentance, and without repentance there is no life (Luke 13:3). (See complete sermon Matthew 5:4:The Blessing of Mourning) (Bolding added)

Newton also has some practical thoughts on how mourning functions in a person's life...

 In conversion - Spiritual mourning begins in conversion; it is the pathway to repentance. It comes as a gift of God’s grace that enables us to see our sin as an offense against God, and to understand the judgment of God that weighs against us. This is where the promise, “for they shall be comforted,” shines. When a person faces his own lost condition before God, and sees his unworthiness of forgiveness, and yet God in His mercy saves him, then you can be assured that he is “comforted!” The word implies that God comes near to him with great consolations. It is not a comfort that leads to cockiness as though he deserved what God did, but a comfort that humbles him, that spills forth in continual gratitude as he is converted to Christ.

In Sanctification - Spiritual mourning continually operates in the life of the Christian. For as he sins it brings grief, and grief causes him to turn to Christ and the sufficiency of His death; then he is comforted again. “Whenever the Christian is conscious of his own sin,” writes Ferguson, “he will be grieved by it” [20]. Grief leads to repentance, and comfort. Watson adds, “The soul of the Christian is most eased when it can vent itself by holy mourning” [76]. It was this same idea that Martin Luther put at the top of his Ninety-five Theses that he nailed to the church door at Wittenberg. “When our Lord and Master Jesus Christ said, “Repent” (Mt 4:17), he willed the entire life of believers to be one of repentance.” Jeremiah Burroughs, another Puritan, offers us great insight on why mourning is part of our sanctification.

As weeds grow very rank in summer time, now in the winter the frost nips the weeds and keeps them under; but if it be a long frost it kills them. And so doth a mournful condition; if it be sanctified, it kills the vermin, it kills our lusts, and is a special means of mortification in the soul; and therefore blessed are they that do mourn, and carry themselves graciously in a mourning condition [The Saints’ Happiness, 38].

In glorification - You can easily see the progress, going from conversion—and the justification that takes place, to sanctification, and finally to glorification. It is not that spiritual mourning follows into glorification, but it is the pathway to it. For the ultimate comfort promised by Christ, “for they shall be comforted,” has a future dimension that points to that time of glorification forever in the Lord’s presence. It is that blessed hope of the Christian, that time in which the mortal shall put on immortality, death will no longer be in the pictured; sin and death will have long been put asunder as enemies under the feet of Christ. And who can describe the measure of Christ’s eternal comfort? (See complete sermon Matthew 5:4:The Bles