THEREFORE YOU ARE TO BE PERFECT AS YOUR HEAVENLY FATHER IS PERFECT: Esesthe (2PFMI) oun humeis teleioi os o pater humon o ouranios teleios estin. (3SPAI): (Genesis 17:1; Leviticus 11:44; 19:2; 20:26; Deuteronomy 18:13; Job 1:1,2,3; Psalms 37:37; Luke 6:36,40; 2 Corinthians 7:1; 13:9,11; Philippians 3:12-15; Colossians 1:28; 4:12; James 1:4; 1 Peter 1:15,16) (Mt 16,45; Ephesians 3:1; 5:1,2; 1 John 3:3)

Therefore - Based on what He has just said about the need to demonstrate in one's life those things He has just described such as going the extra mile (now we know where this common saying comes from!), loving enemies and even praying for them, etc.

C H Spurgeon exhorts all blood bought, heaven bound sons of God to...

Stretch towards the highest conceivable standard, and be not satisfied till you reach it.

You are to be - This phrase is interpreted by many in the sense of a future command. Jesus clearly intends it as an exhortation in light of all He has already said. We are called to be lights in this present darkness and the best light shines like the Father and gives glory to Him. Others see in this statement a promise -- "you shall therefore be perfect" (Young's Literal). The pronoun you is emphatic and clearly charges those who would be His followers to be perfect. 

To be (2076) (estin) is present tense indicating that this is continually to be the goal to which the believer is ever straining and stretching toward in this life. This charge by our Lord, reminds one of Paul's great declaration...

I press on toward the goal for the prize of the upward call of God in Christ Jesus. (see note Philippians 3:14)
 

You are to be perfect - This phrase without a doubt is the most unsettling, even shocking aspect of Jesus’ Sermon on the Mount for if the gauge of righteousness is absolute perfection, what hope is there for anyone? Jesus eliminated all human standards of morality and conduct when He spoke forth these words for even the "righteous" Pharisees, who were generally considered to be the embodiment of the highest human righteousness, could not qualify according to this standard. Jesus however did not create a new standard, for God has never had any other standard for man other than perfect holiness (see Lev 11:44, notes 1 Peter 1:15; 16)

El Shaddai (note) (our Sufficiency) gave a similar charge (to "be ye perfect") to Abraham, Moses recording ...

Now when Abram was ninety-nine years old, the LORD appeared to Abram and said to him, "I am God Almighty (El Shaddai); Walk before Me, and be blameless (tamiym [root means "complete"] = single-hearted, without blame, sincere, wholly devoted to the Lord). (Genesis 17:1)

In Leviticus God declared...

I am the LORD your God. Consecrate yourselves therefore, and be holy; for I am holy. And you shall not make yourselves unclean with any of the swarming things that swarm on the earth. (Leviticus 11:44)

We also see that what Jesus says in His sermon closely parallels what Moses recorded in Deuteronomy...

You shall be blameless (Septuagint translates the Hebrew word tamiym [root that means "complete" and so single-hearted, without blame, sincere, wholly devoted to the Lord. Tamiym is used to describe the Lord in Job 37:16, 2 Sa 22:31. It describes sacrificial animals without blemish or spot - no defects] with teleios the same word Jesus uses in Mt 5:48 for "perfect") before the LORD your God. (Deut 18:13)

Spurgeon alludes to the promise aspect of Matthew 5:48 in his book When Christ Returns, writing that Christ will be glorified for then...

We will be “perfect, even as [our] Father which is in heaven is perfect” (Matthew 5:48).

Truly, beloved, He who works this in us will be a wonder. Christ will be admired and adored because of this grand result. O mighty Master, with what strange moral alchemy did You work to turn that morosely dispositioned man into a mass of love! How did You work to lift that selfish lover of mammon up from his hoarded gains to make him find his gain in You? How did You overcome that proud spirit, that fickle spirit, that lazy spirit, that lustful spirit—how did You contrive to take all these away? How did You exterminate the roots of sin, even the fine root hairs, out of Your redeemed, so that not one tiny filament remains?
(When Christ Returns)

John MacArthur comments that...

Because God is perfect, those who are truly his children will move on in the direction of his perfect standard. f you are stalled, or if you are slipping in the opposite direction, it is right that you examine yourself. Pursuing the standard of perfection does not mean we can never fail. It means that when we fail we deal with it. Those with true faith will fail—and in some cases, fail pathetically and frequently—but a genuine believer will, as a pattern of life, be confessing sin and coming to the Father for forgiveness (1 John 1:9).

Perfection is the standard.
Direction is the test.

If your life does not reveal growth in grace and righteousness and holiness, you need to examine the reality of your faith—even if you believe you have done great things in the name of Christ. (MacArthur, J., F. The Gospel According to Jesus. Grand Rapids, MI: Academic and Professional Books, Zondervan Pub. House )

In his book the Vanishing Conscience, Dr MacArthur makes an interesting statement that applies to the high standard Jesus called us to in Matthew 5:48

It is folly to think that being imperfect somehow provides us with a legitimate excuse to exempt us from God’s perfect standard. (The Vanishing Conscience)

Pentecost writes that now...

our Lord answers the question uppermost in the minds of His hearers,

How good does a man have to be to please God?

Christ’s answer was,

“As good as God is.”

God is not satisfied with anything less than absolute perfection. How good does a man have to be to stand in the presence of God? He has to be as good as God is. If one falls the slightest degree short of the standard of God’s inviolable, unalterable holiness, he is unacceptable to God. What is God’s standard?

“Be ye therefore as perfect as your Father in heaven is perfect.”...

God does not hate; therefore, if a man is as perfect as God is, he will not hate (Mt 5:21-26).

God does not lust; if a man is as perfect as God is, he will not lust (Mt 5:27-32).

God does not deceive; if a man is as perfect as God is, he will not deceive (Mt 5:33-37).

God does not retaliate for injury done; if a man is as perfect as God is, he will not retaliate. He will not stand on his own rights (Mt 5:39-42).

God does not discriminate in His response to needs; if a man is perfect, he will love not only his friends but also his enemies (Mt 5:43-47). These are the marks of a truly godly man.  (Pentecost, J. D. Design for living: Lessons in Holiness from the Sermon on the Mount. Kregel Publications)

Perfect (5046) (teleios from télos = goal, purpose) means one who has attained to the end (aim) or to the intended goal (telos). If anything has fully attained that for which it is designed it is perfect (teleios). Teleios was used to refer to the maturity of an adult, which is the end or aim to which the child points. Thus it denotes those who have attained the full development of innate powers, in contrast to those who are still in the undeveloped state (children). Teleios signifies consummate soundness, and includes the idea of being whole.

Teleios (perfect) is often translated “mature”, but in the context the meaning is that of perfection, because the heavenly Father is the standard.  We must measure ourselves not by others but by the Father. To be perfect is essentially the sum of all that Jesus teaches us in the Sermon on the Mount and ultimately is the sum of all that is taught in Scripture. The “sons” are to be perfect, as their heavenly Father is perfect. That perfection is absolute perfection and includes our words (Matthew 5:33-35, 5:36-37), our responses to injuries (Matthew 5:38-39, 5:40-42), and our dealings with our enemies (Matthew 5:43-45, 5:46-48). The great purpose of salvation, the goal of the gospel, and the great desire of the Father is that men would be conformed to the image of His Son, the only One Who ever lived a perfect life. If a man could live the way Jesus has told us to in this chapter, he would truly be perfect.

Perfection is our continual goal in this life, but in the life to come will be our everlasting possession and experience.

A T Robertson explains that perfect...

comes from telos, end, goal, limit. Here it is the goal set before us, the absolute standard of our Heavenly Father. The word is used also for relative perfection as of adults compared with children.

Such perfection in this present earthly life is clearly impossible in one’s own power. To those who wonder how Jesus can demand the impossible, He later says,

With men this is impossible, but with God all things are possible (Mt 19:26, Mk 10:27) (IM-possible but HIM-possible!)

What God commands and demands, He empowers and enables that it might be accomplished. As one person has phrased it...

You can't. He never said you could.
But He can and He always said He would.

For believers, it is Christ in us the hope of Glory (see note Colossians 1:27) Who empowers us by His indwelling Spirit (see notes Galatians 5:16; 17; 18; 24; cp notes Romans 8:13) to live the way the Father commands. Christ is now our life (see note Colossians 3:4). As one person has noted, if you remove the "a" from "Christian" and place it at the front what you see is "A Christ In"!

Paul beautifully expressed the "divine tension" of God's sovereignty and man's responsibility in his letter to the Philippians calling on them to...

work out (present imperative = command to do this continually) your salvation (this is our responsibility) with fear and trembling, (but then he explains how this is even possible)  for it is God Who is at work in you (continually - present tense), both to will (He continually [present tense] gives us even the "want to" so to speak) and to work (He continually [present tense] energizes the working out of our salvation) for His good pleasure (this is the goal - the Father's good pleasure - pleasing to Him). (See notes Philippians 2:12; 2:13)

This same principle is even seen in the Old Testament where God promises the New Covenant to Israel (and by way of application to all who by grace through faith enter the New Covenant)...

"Moreover, I will give you a new heart and put a new spirit within you; and I will remove the heart of stone from your flesh and give you a heart of flesh. And I will put My Spirit within you (God's provision is His Spirit) and cause you to walk in My statutes (God's power), and you (man's responsibility) will be careful to observe My ordinances. (Ezekiel 36:26-27)

In Colossians Paul illustrated this divine dynamic in his own life writing...

And we proclaim Him (Christ in us the hope or absolute assurance of glory), admonishing every man and teaching every man with all wisdom, that we may present every man complete (teleios - fully mature) in Christ. For this purpose (to present every man complete or fully mature in Christ) also I labor (present tense = continually working to the point of exhaustion) striving (present tense = continually agonizing) (How was [and still is] this possible?) according to His power (energeia - God's effectual working in believers) which mightily works (present tense = continually) within me (See notes Colossians 1:28; Colossians 1:29)

Paul reminded the saints at Colossae that Epaphras was praying for them...

Epaphras, who is one of your number, a bondslave of Jesus Christ, sends you his greetings, always laboring earnestly for you in his prayers, that you may stand perfect (teleios) and fully assured in all the will of God. (see note Colossians 4:12)

In Hebrews we see this "tension" in the great prayer...

Now the God of peace, Who brought up from the dead the great Shepherd of the sheep through the blood of the eternal covenant, even Jesus our Lord, equip you (God's provision) in every good thing to do (man's part) His will, working in us (God's continual working) that which is pleasing in His sight, through Jesus Christ, to Whom be the glory forever and ever. Amen (See notes Hebrews 13:20; 21)

Regarding being perfect, Paul wrote...

Not that I have already obtained it, or have already become perfect (teleioo - accomplished or brought to an end or to the intended goal - which ultimately will be when we are glorified), but I press on (dioko = a verb which gives the picture of pursuing with earnestness and diligence in order to obtain a goal. The present tense = this was Paul's habitual practice, the warp and woof of his life) in order that I may lay hold of that for which also I was laid hold of by Christ Jesus (to be perfect as our heavenly Father is perfect - when we see Christ we shall be like Him, and this will be the final and consummate fulfillment of His call for us to be perfect.)
13 Brethren, I do not regard myself as having laid hold of it yet; but one thing I do: forgetting what lies behind and reaching forward to what lies ahead,
14 I press on (dioko = a verb which gives the picture of pursuing with earnestness and diligence in order to obtain a goal. The present tense = this was Paul's habitual practice, the warp and woof of his life) toward the goal for the prize of the upward call of God in Christ Jesus.
15 Let us therefore, as many as are perfect (teleios = signifies consummate soundness and includes the idea of being hold. In believers the idea is not sinless perfection but full spiritual maturity), have this attitude; and if in anything you have a different attitude, God will reveal that also to you;
16 however, let us keep living by that same standard to which we have attained. (See notes Philippians 3:12; 13; 14; 15; 16)

Paul knew that his record had already been cleared before God on the basis of his faith in Christ and in that sense he was perfect (positionally) but he was also aware of the practical work of his being perfected in this present life and the future prospect of absolute perfection in the life to come writing that...

our citizenship is in heaven, from which also we eagerly wait for a Savior, the Lord Jesus Christ Who will transform the body of our humble state into conformity with the body of His glory, by the exertion of the power that He has even to subject all things to Himself. (See notes Philippians 3:20; 21)

At the risk making Jesus' call to be perfect as your heavenly Father is perfect look too simplistic or too "mechanical", the table below attempts to demonstrate the three aspects of Christian perfection...
 

(See related discussion - Three Tenses of Salvation)

What should motivate us to be perfect as our heavenly Father is perfect? Paul exhorts the saints in Corinth...

Therefore, having these promises (read them in the preceding passages 2Cor 6:14-18) beloved, let us cleanse ourselves from all defilement of flesh and spirit, perfecting (epiteleo = fulfill completely. Bring about a result according to an objective.  Present tense = our continuing practice) holiness in the fear of God. (2 Corinthians 7:1)

How can we be sure that we will attain the state of perfection Jesus calls us to? Again the apostle Paul would seem to address this declaring to the saints at Philippi...

For I am confident of this very thing, that He who began a good work in you will perfect (epiteleo) it until the day of Christ Jesus  (See note Philippians 1:6)

Comment: The day of Christ Jesus refers to the glorious day of His Second Coming when faith shall become sight, and then face to face what we now I know in part, then we shall know fully just as we also have been fully known.)

Henry Morris explains Jesus' call to be perfect this way...

Jesus knew no believer could be sinlessly perfect (see notes Matthew 6:14; 15 "transgressions") in this life. Nevertheless, this must be the standard and the goal--not for gaining salvation, but for living the Christian life. The word perfect also can be understood as "complete" or "fully mature," but this state is no more attainable than sinless perfection--in fact, they are really the same. We do have such a standing in Christ, and we should perpetually seek to fulfill this standard by God's help. (Morris, Henry: Defenders Study Bible. World Publishing)

Harry Ironside writes that...

This is perfection in the sense of the complete absence of partiality, thus imitating Him who is no respecter of persons (Acts 10:34), but who lavishes His favors on just and unjust alike. God's choicest blessings are for those who demonstrate the same spirit of reverence for Him and meekness and compassion for others, which were seen in all their fullness in our blessed Lord as He walked this earth in the days of His flesh (see note Hebrews 5:7). That spirit, which is beyond the reach of the natural man, is fulfilled in those who have received a new life and nature through trusting in Christ as their Savior. No adverse circumstances can disturb the serenity of those who know the Lord and who acknowledge His authority over their lives.

Surely every right-thinking person must admit that the righteousness inculcated by our Lord in this matchless discourse (which has won the admiration of intelligent people everywhere) is a standard far beyond that to which the natural man can attain. It is only when one has been born again that he can live on this high plane. When men talk of the Sermon on the Mount being religion enough for them, they only show how little they understand the meaning of our Master's words. He portrays a supernatural life that can be lived only by supernatural power--the power that the Holy Spirit gives to him who believes the gospel. (Commentary on Matthew - Harry Ironside) (Bolding added)

Wiersbe writes that...

The word perfect in Matthew 5:48 does not imply sinlessly perfect, for that is impossible in this life (though it is a good goal to strive for). It suggests completeness, maturity, as the sons of God. The Father loves His enemies and seeks to make them His children, and we should assist Him! (Wiersbe, W: Bible Exposition Commentary. 1989. Victor or Logos)

Spurgeon has this devotional thought on "If we walk in the light, as he is in the light" (1 John 1:7) writing...

As He is in the light! Can we ever attain to this? Will we ever be able to walk as clearly in the light as He whom we call “Our Father” is, of whom it is written, “God is light, and in him is no darkness at all” (1 John 1:5)? Certainly, this is the model which is set before us, for the Savior Himself said, “Be ye therefore perfect, even as your Father which is in heaven is perfect” (Matt. 5:48). Although we may feel that we can never rival the perfection of God, we are yet to seek after it and never be satisfied until we attain to it. (Daily Help)

Spurgeon commenting on Matthew 5:48 helps us understand what it means to be perfect...

Rise out of ordinary manhood. Get beyond what others might expect of you. Have a high standard. “Be ye, therefore, perfect, even as your Father which is in heaven is perfect.”

Stretch towards the highest conceivable standard, and be not satisfied till you reach it.

We should reach after completeness in love,- fullness of love to all around us. Love is the bond of perfectness; and if we have perfect love, it will form in us a perfect character. Here is that which we aim at,-perfection like that of God; here is the manner of obtaining it,-namely, by abounding in love; and this suggests the question of how far we have proceeded in this heavenly direction, and also the reason why we should persevere in it even to the end, because as children we ought to resemble our Father.

Scriptural perfection is attainable, it lies rather in proportion than in degree. A man’s character may be perfect and entire, wanting nothing; and yet such a man will be the very first to admit that the grace which is in him is at best in its infancy, and though perfect as a child in all its parts, it has not yet attained to the perfection of full-grown manhood.

What a mark is set before us by our Perfect King, who, speaking from his mountain-throne, saith, “Be ye perfect, even as your Father which is in heaven is perfect”!

Lord, give what Thou dost command; then both the grace and the glory will be Thine alone. (Amen)

In his sermon (God's Time for Comforting) Spurgeon writes...

Though you cannot be perfect, yet you must want to be perfect, and there must not be any sin which you knowingly spare. Cut them in pieces, every one of them; as soon as you know that anything is wrong, I pray you to have such a tender conscience that you will seek to escape from it; for, as long as you harbor even one of them, comfort will never come to you.

Another sermon Spurgeon (on Colossians 2:6 - see note) makes a comment that relates to be perfect...

Well, then; keep there! Keep there! Never get an inch beyond that position. When you get sanctified, still look to Christ as if you were unsanctified. When you are on the verge of being glorified, look to Him as if you were just newly come out of the hole of the pit. Hang upon Christ, you who are the best, just as though you were the worst. The same faith which saved Mary Magdalene, which saved Saul of Tarsus, must save you in the moment, when you shall be the nearest to the perfect image of Christ Jesus. It is “none but Jesus” now to your soul; let it be “none but Jesus, — none but Jesus,” as long as you live. (See full sermon by Spurgeon - Colossians 2:6 Walk for a Time to Come)

Ryrie explains that regarding His call to be perfect Jesus does not mean...

without sin, but mature and complete in the likeness of God. (The Ryrie Study Bible: New American Standard Translation: 1995. Moody Publishers)

Criswell says...

The idea of gaining maturity does not fully interpret this quotation from Lev. 19:2. Perfection is attainable only when evil is vanquished and the kingdom citizen is glorified. His present life ought to exhibit maturity and progress toward that ultimate goal of perfection. Whatever the interpretation, the demand for perfection is not to be weakened. Rather, Matthew magnifies the fact that the righteousness demanded of kingdom citizens cannot be attained on the basis of merit, but must be given through mercy. This statement summarizes Mt 5:17-48. (Criswell, W A. Believer's Study Bible: New King James Version. 1991. Thomas Nelson)

James Montgomery Boice explains that in order to be perfect as your heavenly Father is perfect...

you must turn away from your own efforts completely and receive instead the perfection which God has already taken steps to provide for you. Nothing that you will ever do will be perfect. Only what God does is perfect. Hence, if you are to reach the perfection which God requires, it must be as the result of His working for you and in you...

In the eighteenth Psalm David speaks of perfection twice, once of God’s perfection and once of man’s. The point of the verses it that God is responsible for both kinds. In Psalm 18:30 David writes, “As for God, his way is perfect.” Then two verses farther on he adds, “It is God who arms me with strength and makes my way perfect” (Ps 18:32). Who is God? God is the One who is perfect. What does he do? God works to perfect sinful men. (Sermon on the Mount) (Bolding and underlining added for emphasis)
 

MacDonald explains that Jesus'...

word perfect must be understood in the light of the context. It does not mean sinless or flawless. The previous verses explain that to be perfect means to love those who hate us, to pray for those who persecute us, and to show kindness to both friend and foe. Perfection here is that spiritual maturity which enables a Christian to imitate God in dispensing blessing to everybody without partiality. (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson or Logos)

The Preacher's Commentary explains that...

The commandment to be perfect in the same way our Father is perfect is to be seen in this context of love. His perfect love is without discrimination; our love is to be open to all people, friends and enemies. We should not read the word “perfect” as a philosophical or moralistic perfection. As Bruce Larson has said of moralistic perfectionism,

“It will drive you up the wall. You can’t be all right and be well!”

The Greek word is teleios, a word which carries the meaning of culmination, of maturity, of achievement in function. We were created in the image of God, and having perverted this image in sin we are re-created in grace and predestined “to be conformed to the image of his Son” (see note Romans 8:29, kjv). The goal of the disciple is to live in this perfection, this teleios, this extension of the love of Christ. It is an active word and, in context, means that our love must be all-inclusive as God’s is all-inclusive. (Augsburger, M. S & Ogilvie, L. J. The Preacher's Commentary Series. Thomas Nelson or