THEREFORE BRETHREN: dio mallon, adelphoi:

Why the "therefore"? Because of our provision (everything pertaining to life and godliness" "His precious & magnificent promises") and our ''potential'' ("partakers of the divine nature"). In fact this verse closely parallels Peter's exhortation to diligence in (v5).

"Therefore" ties these great truths together.

Peter is saying that on the basis of everything I have said

"Therefore, brethren be all the more diligent..."

In this verse and the next Peter gives the 2 results of spiritual growth, the first relating to this present life (assurance of salvation) and the second to the future (abundant entrance into God's eternal kingdom)

Though God is “sure” who His elect are and has given them an eternally secure salvation (see notes 1 Peter 1:1;  1:2; 1:3; 1:4; 1:5; cf. see note Romans 8:28ff) (click for John MacArthur's 8 Sermon series on assurance of salvation  and scroll down to "Reasons People Lack Assurance" and "Tests of Assurance), believers often do not have assurance of their salvation. Security is the Holy Spirit revealed objective fact that salvation is forever (see note Romans 8:16). Assurance is one’s (subjective) confidence that he or she possesses eternal salvation. In other words, believers who pursue the spiritual qualities delineated by Peter guarantees to themselves by the fruit God brings forth through them that they are called and chosen (elect) by God unto salvation.

BE ALL THE MORE DILIGENT: mâllon spoudasate (2 PPAAM): (See Torrey's Topic "Diligence")

Diligent (4704) (spoudazo from the noun spoude Peter used earlier in v5) (Click study of related noun spoude or here for verb spoudazo) means to do something in a hurry with intense effort and motivation or involving earnest application to some specific pursuit. In using this word Peter is conveying a sense of urgency and eagerness.

Spoudazo is aorist active imperative which commands a definitive action and conveys a sense of urgency. Make certain of His calling now. Don't put this off!

Spoudazo calls for an intense effort and an eagerness of spirit applied to the believer's walk. This effort is important as it will solidify their sense of assurance that the individual is truly a child of God and a member of His family.

The writer of Hebrews conveys a parallel thought (Heb 6:11) expressing the

"desire that each one of (his readers) show the same diligence so as to realize the full assurance of hope until the end".

Are you struggling with lack of assurance that you are genuinely saved, delivered from the wrath to come? Peter is writing a great prescription for what ails you.

TO MAKE CERTAIN: poieisthai (PMN) bebaios: (See Torrey's Topic "Assurance")  

Make (4160) (poieo) means make or do. Poieo is in the present tense which calls for continuous effort -- making certain (strengthening our assurance of salvation) is to be a lifelong process, and as such is synonymous with progressive sanctification (holiness). Poieo is also in the middle voice which calls for the reader to personally initiate this action and to participate in the effects of the development of the virtues leading to holiness. Peter is saying "make certain for yourself". So if we are diligently supplying these qualities, and they are increasing, we can know that we have salvation and can avoid the awful struggle of doubt and fear associated with a lack of assurance.

Wayne Grudem writes that...

The way that we confirm our call and election, then, is to continue to grow in “these things.” (Ed: The "things" mentioned in 2Pe 1:5; 1:6; 1:7 - see notes) This implies that our assurance of salvation can be something that increases over time in our lives. Every year that we add to these character traits in our lives, we gain greater and greater assurance of our salvation. Thus, though young believers can have a quite strong confidence in their salvation, that assurance can increase to even deeper certainty over the years in which they grow toward Christian maturity. If they continue to add these things they will confirm their call and election and will “never fall.” (Grudem, W: Systematic Theology: An Introduction to Biblical Doctrine. IVP; Zondervan, 1994 or Logos) (Bolding added)

Certain (949) (bebaios from baino = to go, walk, step) describes that which is fixed, stable, sure, attested to and certified. It is something which is unwavering and persistent and thus can be relied on or depended on. It pertains to that which is known with certainty. It refers to something that has validity over a period of time (e.g., the promise made to Abraham remained valid to NT believers, see note Romans 4:16).  Figuratively bebaios refers to that upon which one may build, rely or trust.

Bebaios is something that can be relied on not to cause disappointment for it is reliable and unshifting. In practice, though not originally, bebaios is close to pistos  (4103) (trustworthy, dependable, reliable, faithful)

Bebaios is used 9 times in the NT (Ro; 2Co; Heb 5x; 2 P 2x) and is translated a variety of ways in the NASB: certain, 1; firm, 2; firmly grounded, 1; guaranteed, 1; more sure, 1; steadfast, 1; unalterable, 1; valid, 1.

Romans 4:16 (note) For this reason it is by faith, that it might be in accordance with grace, in order that the promise may be certain to all the descendants, not only to those who are of the Law, but also to those who are of the faith of Abraham, who is the father of us all,

2 Corinthians 1:7 and our hope for you is firmly grounded, knowing that as you are sharers of our sufferings, so also you are sharers of our comfort.

Hebrews 2:2 (note) For if the word spoken through angels proved unalterable, and every transgression and disobedience received a just recompense,

Hebrews 3:14 (note) For we have become partakers of Christ, if we hold fast the beginning of our assurance firm until the end;

Hebrews 6:19 (note) This hope we have as an anchor of the soul, a hope both sure and steadfast and one which enters within the veil,

Hebrews 9:17 (note) For a covenant is valid only when men are dead, for it is never in force while the one who made it lives.

2 Peter 1:10 (note) Therefore, brethren, be all the more diligent to make certain about His calling and choosing you; for as long as you practice these things, you will never stumble;

2 Peter 1:19 (note) And so we have the prophetic word made more sure, to which you do well to pay attention as to a lamp shining in a dark place, until the day dawns and the morning star arises in your hearts.

TDNT says that bebaios

means “standing firm on the feet,” “steadfast,” “maintaining firmness or solidity,” “steadfast for …” Hence “firm” in the sense of having inner solidity. In respect of abstract things and persons bebaios thus comes to mean “steady,” “sure,” “reliable” “steadfast,” or “certain. " (Kittel, G., Friedrich, G., & Bromiley, G. W.  Theological Dictionary of the New Testament. Eerdmans)

Bebaios has a legal sense, signifying a legal guarantee, obtained by the buyer from the seller, to be gone back upon should a third party claim the thing. Thus in classic Greek bebaios described a warranty deed somewhat like a guarantee one might have today on an automobile or similar product.  A holy life is like a "guarantee" demonstrating one's calling and election to others as well as to one's self.

Peter uses bebaios describing the Word of God, writing that

we have the prophetic word [made] (not in Greek. Literally = "word more sure")  more sure, to which you do well to pay (close) attention (nautical term that meant to hold a ship in a direction and so to sail towards!) as to a lamp shining in a dark (miry, filthy, murky, dismal, dark) place, until the day dawns (shines through, breaks forth) and the morning star arises in your hearts. (see note 2 Peter 1:19)

What Peter is saying (although the translations in some versions make this meaning difficult to discern) is not that the eyewitness account of Christ's majesty at the transfiguration confirmed the Scriptures, but that the prophetic word is a more reliable attestation or verification of the teachings about the person, atonement, and second coming of Christ than even the genuine first hand experiences of the apostles themselves. Courson has an interesting comment on this passage adding that

If someone offered you the choice of either being on Mount Hermon with Jesus, seeing Moses and Elijah, hearing a voice from heaven—or having the Old Testament, most of us would choose to see the Lord glowing, to see Moses and Elijah, to hear a voice from heaven. But Peter would choose otherwise. Why? Because experiences fade, but the Word endures. The problem with experiences is that all they produce is a craving to see more...Having been around for a while, I would rather hear a great Bible study and be fed from the Scriptures than see a bunch of experiences unfolding. There was a time when this was not true in my life. But the longer I walk with the Lord, the more I realize that experiences fade—even the valid ones, even the wondrous ones. Only the Word endures. (Courson, J.  Jon Courson's Application Commentary. Page 1589. Nashville, TN: Thomas Nelson)

The writer of Hebrews uses bebaios reminding his Jewish readers, some of whom were teetering on going back to Judaism, that in Christ we have a hope set before us and

This hope we have as an anchor (that which forms a bend i.e., an anchor and can stabilize the thing to which it is attached) of the soul, a hope both sure (does not totter, cannot be thrown down, steady, immovable, safe, secure from peril) and steadfast (bebaios) and one which enters within the veil, where Jesus has entered as a forerunner for us, having become a high priest forever according to the order of Melchizedek." (see notes Hebrews 6:19; 6:20) (Comment: Vincent says means here a hope that sustains one’s steps in going, one that does not break down under what steps upon it)

MacDonald comments that

"In the storms and trials of life this hope serves as an anchor of the soul. The knowledge that our glorification is as certain as if it had already happened keeps us from drifting on the wild waves of doubt and despair. The anchor is not cast in the shifting sands of this world but takes hold in the heavenly sanctuary. Since our hope is the anchor, the meaning is that our hope is secured in God’s very Presence behind the veil. Just as sure as the anchor is there, we shall be there also." (MacDonald, W & Farstad, A. Believer's Bible Commentary: Thomas Nelson)

Bebaios was used of confirming something as in the legal terminology of validating a will. So a Christian by growing in grace becomes assured (stabile, secure in the salvation, having assurance of their salvation) of having been called and elected by God. This stresses the responsibility of the believer to live in conformity to his calling into a partaking of the divine nature in Christ Jesus (v4).

The exhortation is that the believer should make sure of the fact that he is saved by seeing to it that the Christian graces superabound in his life. There is no idea here of making sure that we retain our salvation but that we possess salvation.

Spurgeon comments that...

"Full assurance is an excellent attainment. It is profitable for a man to be certain in this life, and absolutely sure of his own calling and election. But how can he be sure? Now, many of our more ignorant hearers imagine that the only way they have of being assured of their election is by some revelation, some dream, and some mystery. I have enjoyed very hearty laughs as the expense of some people who have trusted in their visions. Really, if you had passed among so many shades of ignorant professing Christians as I have; and had to resolve so many doubts and fears, you would be so infinitely sick of dreams and visions that you would say, as soon as a person began to speak about them, "Now, do just hold your tongue." "Sir," said a woman, "I saw blue lights in the front parlor when I was in prayer, and I thought I saw the Saviour in the corner, and I said to myself I am safe."

Peter is not necessarily urging the readers to engage in more strenuous activities per se. A believer's spiritual growth confirms that God has called and chosen him. The "blighted" condition pictured in v9 destroys such personal assurance.

In 1654 Thomas Brooks wrote the following statement regarding the believer's assurance of salvation...

ABOUT HIS CALLING: ten klêsin: (Ro 1:1, 7, 8:28-30; 8:30, 9:24; 1Cor 1:2, 1Cor 1:9; 1Cor 1:24, 1Cor 1:26, Gal 1:6; Eph 1:18, 4:1, 1Thes 2:12; 2Thes 2:14; 2Ti 1:9; Heb 3:1; 1Pet 2:9,21;2 Pet 1:10;Jude 1:1, Rev 17:14, Torrey's Topic Call of God)  

Note that "calling and choosing" are modified by a single definite article (ten) and thus are viewed as intimately related for our salvation.

Calling  (2821) (klesis)  means a call and was used for an invitation to a banquet. In the NT the word is used metaphorically of the call or invitation to come into the kingdom of God with all its privileges. Here "klesis" refers to the divine call by which Christians are introduced into the privileges of the gospel. God’s invitation (klesis) to man to accept the benefits of His salvation is what this calling is all about, particularly in the gospels. It is God’s first act in the application of redemption according to His eternal purpose (Ro 8:28). A distinction is made between God’s calling and men’s acceptance of it (Mt 20:16).

Klesis is used 11 times in the NT in the NASB (Ro; 1Co 2x; Eph 3x; Phil; 2Th; 2Ti; Heb; 2 P)  (Click study of related word kletos, and a discussion of who are "the called")

Romans 11:29 (note) for the gifts and the calling of God are irrevocable.

1 Corinthians 1:26 For consider your calling, brethren, that there were not many wise according to the flesh, not many mighty, not many noble;

1 Corinthians 7:20 Let each man remain in that condition in which he was called.

Ephesians 1:18 (note) I pray that the eyes of your heart may be enlightened, so that you may know what is the hope of His calling, what are the riches of the glory of His inheritance in the saints,

Ephesians 4:1 (note) therefore, the prisoner of the Lord, entreat you to walk in a manner worthy of the calling with which you have been called,

Ephesians 4:4 (note) There is one body and one Spirit, just as also you were called in one hope of your calling;

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

2 Thessalonians 1:11 To this end also we pray for you always that our God may count you worthy of your calling, and fulfill every desire for goodness and the work of faith with power;

2 Timothy 1:9 (note) who has saved us, and called us with a holy calling, not according to our works, but according to His own purpose and grace which was granted us in Christ Jesus from all eternity,

Hebrews 3:1 (note) Therefore, holy brethren, partakers of a heavenly calling, consider Jesus, the Apostle and High Priest of our confession.

2 Peter 1:10 (note) Therefore, brethren, be all the more diligent to make certain about His calling and choosing you; for as long as you practice these things, you will never stumble;

Klesis can also refer to a call unto Christian service or ministry. That the calling is to more than a Christian profession is clear from the experiences which Paul associates with it. (see note Romans 1:1) No one can be a chosen one unless he is a called one. The initiative always comes from God.

Louw Nida defines klesis as an

urgent invitation to someone to accept responsibilities for a particular task, implying a new relationship to the one who does the calling; the station in life or social role which one has."

Vine says klesis

a calling, is always used in NT of that calling the origin, nature and destiny of which are heavenly (the idea of invitation being implied); it is used esp of God's invitation to man to accept the benefits of salvation.

In the present context klesis refers to those who have been summoned by God (the following phrases are meant to be read as one long sentence which gives a Biblical statement regarding calling)...

"according to His purpose" (see note Romans 8:28) to salvation (see note Romans 8:30),

"saints by calling" (1Cor 1:2),

"both Jews and Greeks" (1Cor 1:24),

having been called "with a holy" (see note 2 Timothy 1:9),

"heavenly calling" (see note Hebrews 3:1)

"out of darkness into His marvelous light" (see note 1 Peter 2:9)

"by grace" (Gal 1:6)

"not from among Jews only, but also from among Gentiles" (see note Romans 9:24)

through the "gospel" that we "may gain the glory of our Lord Jesus Christ" (2Th 2:14)

and be brought "into fellowship with His Son, Jesus Christ our Lord" (1Cor 1:9)

and return in triumph "with Him" at the end of this age (see note Revelation 17:14).

God's great doctrine of our calling should cause all the "called of Jesus Christ" to exclaim "Glory!"

While God’s choice of the elect is firm and certain in God (see note 2 Timothy 2:9), it may not always be obvious to the individual Christian.

McGee summarizes Peter's command explaining that...

In other words, the security of the believer is objective; it is something that cannot be disturbed. However, your assurance can certainly be disturbed by the life you live. If your life is not lived in sincerity and truth, you are bound to lie on your bed at night and wonder if you really have been born again. While it is true that Christ has done everything necessary to save you and keep you saved, your Christian life to be meaningful is something that you have to work at.

Who are the CALLED? Well, they are those who have heard. The Lord Jesus made it clear when He said,

"My sheep hear My voice, and I know them, and they follow Me” (Jn 10:27).

If you are following someone or something else, you haven’t heard Him, you are not one of His sheep. The ones who hear and follow Him are the called ones. Let’s not argue about election. It is as simple as this: He calls, and you answer. If you have answered, you are among the elect, one of “the called of Jesus Christ.” Paul assures the Roman Christians that they are called ones. In the writings of both Paul & Peter when they mention "called" ("call", "calling", etc), the reference is to an "effectual" call, that is a call which is answered & thus "the called" equates essentially with those who are "the chosen" or "the elect".

Note that the gospels use the term called differently -- in (Mt 22:1-13,14) many were "called" to the "wedding feast" but few were "chosen", so in the gospels the term "call..." was not synonymous with an effectual call to salvation.

Spurgeon makes the distinction between "general" and "special" calling writing that...

By the word "calling" in Scripture, we understand two things—one, the general call, which in the preaching of the gospel is given to every creature under heaven; the second call (that which is here intended) is the special call—which we call the effectual call, whereby God secretly, in the use of means, by the irresistible power of his Holy Spirit, calls out of mankind a certain number, whom he himself hath before elected, calling them from their sins to become righteous, from their death in trespasses and sins to become living spiritual men, and from their worldly pursuits to become the lovers of Jesus Christ."

Peter pointed out that “calling” and “election” go together. The same God who elects His people also ordains the means to call them. The two must go together, as Paul wrote to the Thessalonians

God has chosen you from the beginning for salvation through sanctification by the Spirit and faith in the truth...It was for this He called you through our gospel,” (2Th 2:13,14).