“You therefore beloved, knowing this beforehand, remain vigilant, lest you, being carried off by error of wicked men, fall from your own steadfastness. So, keep growing in grace and knowledge of our Lord and Savior Jesus Christ. To Him be the glory both now and to the day of eternity.”
The beginning of v. 17 includes the final use of ἀγαπητοί (beloved) in the letter and thus concludes the final section of encouragement. Peter’s use of ὑμαεῖς (you/y’all) is somewhat emphatic and makes a nice contrast between the readers now being addressed and those who are damned for their twisting of Scripture (vv. 14-16) while οὖν (therefore) draws an inference from that grim preceding section.
As one might expect, the character of this final and brief paragraph is much like Peter’s introduction. We have already noted Peter’s returned use of hendiadys (vv. 14&16) and will see it again shortly (v. 18). The conclusion of grace (χάρις) seems to mirror the introduction (1:2), which would include the mention of knowledge (ἐπίγνωσις/γνῶσις).  It appears that we have here something of an inclusio.
Peter’s conclusion is masterful, poignant, and precise. In two short verses he reviews the entirety of his letter in summary fashion before concluding with praise. His message is so particularly articulated that there is no chance of him being misunderstood. That clarity is necessary because here Peter summarizes the two main points of his letter.
Summary of Peter’s Message (vv. 17-18a)
It’s worth noting that Peter has changed from using aorist imperatives (vv. 14-16) to present imperatives. The sense is no longer “do it!” but is now “keep doing it.” There is an implication that the saints of Asia Minor are already obeying these commands and Peter is simply encouraging them to continue doing so. This makes a lot of sense when we remember that the entire letter is a reminder of things already taught (1:12; 3:1-2) and that these imperatives briefly and beautifully summarize the entire letter. Looking at the most recent material first, Peter summarizes the warning of chapter 2 with the imperative to remain vigilant (φυλάσσεσθε) before recalling the exhortations of chapter 1 in the command to continue growing (αὐξάνετε).
Guard against Apostasy (v. 17)
“You therefore beloved, knowing this beforehand, remain vigilant, lest you, being carried off by error of wicked men, fall from your own steadfastness.”
While we may be tempted to jump immediately to the imperative (φυλάσσεσθε/remain vigilant), we must first address the present causative participle (προγινώσκοντες/knowing). Peter returns to the earlier theme of knowledge (1:5, 6, 20; 3:3). The prefix preposition προς (from where we get our English term prognosis) indicates a knowledge that comes before the event occurs. It carries the idea of a warning in the present context. But what did the readers know beforehand? The nearest idea in the context is the fact that untrained and unstable men will come to twist this letter just as they have done to Paul’s letters and the rest of Scripture (v. 16). Peter does not say that because of his letter they now know these things but that as he writes they already know these things. How could they know these things unless the Old Testament prophets and the New Testament apostles have already preached concerning these things? Once again, Peter assumes that his readers are familiar with the writings of the prophets and the teaching of the apostles. They are now armed with this foreknowledge, but that foreknowledge assumes action. Because they know this, they are commanded to remain vigilant.
The present imperative indicates an ongoing action more so than the initiation of an action. In other words, the sense is that these readers are already vigilant, or guarding (φυλάσσεσθε) the church and themselves from the possibility of false teaching. The ministry is not a passive occupation where shepherds wait for the wolves to strike before dealing with them. They are to remain alert and watchful so that they see them coming. Yet Peter indicates more than only an alertness for false teachers here. The imperative uses the middle voice which indicates that the readers should be alert to internal dangers as well as external. By all means, watch for wolves attempting to enter the fold. But do not neglect to watch over your own heart and see that you remain in the faith (2 Cor. 13:5). The consequences for a lax guard are steep and irreversible.
The negative subjunctive clause with ἵνα communicates a negative possible consequence, one to be avoided (so that you might not…). That idea is best translated with the English “lest.” Failure to guard one’s own heart will result in a fall from one’s own steadfastness. The subjunctive verb comes from the same root (ἐκπίπτω) used to describe a ship running aground (Acts 27:17, 26, 29) as well as mining out the implications of living according to the law; i.e., falling from grace (Gal. 5:4). Peter is describing apostasy. Failure to maintain an active guard over one’s theology and practice results in being blown off course and being smashed upon the rocks. How could this come about?
Both the Greek and the English text presents the means of falling before mentioning the falling itself. It is by means of being carried off by the error of wicked men. The term translated as wicked (ἄθεσμος) repeats Peter’s description of the men of Sodom (2:7). Their error or false teaching carries people off course and is the means of their destruction. The aorist participle from συναπάγω (carried/lead away/astray) is the same term used by Paul to describe the fact that even Barnabas was carried off by the schemes of the Judaizers in Galatia (Gal. 2:13). Is it possible that Peter here admits that he too was once carried off by false teaching? Remaining vigilant is not optional. A slack guard carries disastrous consequences. In a brief command, Peter summarizes the entirety of chapter 2. You know who is coming, what they will say, and what kind of men they are. You already know what the results of listening to them will be. Keep vigilant. Stay alert.
Grow in Christlikeness (v. 18a)
“So, keep growing in grace and knowledge of our Lord and Savior Jesus Christ.”
Just as the first command summarizes chapter 2, this second imperative (and the last exhortation of the book) summarizes chapter 1. What we see here is something of an inclusio in relation to 1:2 with grace, knowledge, and the Lord Jesus. We also have a final example of hendiadys in the phrase grace and knowledge (χάριτι καὶ γνώσει). Recognizing this phrase as hendiadys determines how we understand the phrase “of our Lord and Savior Jesus Christ.” The genitive τοῦ κυρίου ἡμῶν (of our Lord) must carry the same meaning for both grace (χάριτι) and knowledge (γνώσει). It would be awkward for the phrase to mean “grace from our Lord and knowledge about our Lord.” The single genitive modifies both nouns because they are used together to create a hendiadys.
It is most natural to consider τοῦ κυρίου ἡμῶν as a subject genitive that points to our Lord and Savior who is Jesus Christ as the source from which both grace and knowledge flow. Grace (χάρις) indicates the unmerited favor of God, the bestowal of blessing in place of a curse. This is one of God’s premium characteristics that He revealed to man (Ex. 34:6). The New Testament most often uses this term in conjunction with God’s gracious gift of regeneration, faith, and repentance. At first glance it may seem odd to pair this term with knowledge (γνῶσις) yet the knowledge of Christ is always the initial manifestation of saving grace. One cannot confess Jesus as Lord unless one knows that He is both Lord and Savior (Rom. 10:9-17). In this sense, it is easy to understand how both grace and knowledge come from our Lord and Savior who is Jesus Christ. This hendiadys uses these terms to complete the single idea of salvation, but salvation in the sense that Peter always uses it: the fully developed and climatic achievement of God planned in eternity past, initiated at the point of regeneration, continued throughout this sojourn, and completed in glory. Thus, the command to keep growing in this grace and knowledge.
The verb to grow (αὐξάνω) describes an action that happens to a subject outside of the subject’s control. Yet Peter here puts the verb into an imperative form indicating that his readers must engage their wills to achieve this growth. How does that work? No man has grown physically by willing it. Neither does one grow spiritually simply by force of will. How then can Peter command growth? The image of physical growth is helpful here.
God alone determines the extent of our physical growth. Yet, we are able to impact that growth by providing our bodies with the right kinds of nutrition that promote growth and refrain from introducing substances that inhibit growth. The same is true of our spiritual growth. God alone causes the growth, and yet we can promote or retard that growth by what we take in and/or refrain from. Is this not a restatement of 1:5-11? Yes, God causes the growth, but that does not make spiritual growth an option. In fact, if God causes the growth, then the lack of growth cannot be attributed to His fault. Spiritual maturity is not optional. Again, Peter uses the present imperative as an encouragement. His readers are already engaged in this behavior. Peter simply commands that they continue what they have begun.
Spiritual maturity is not a race in which we compete against other believers. There is no prize for arriving at the finish line first. In fact, speed is never mentioned in the New Testament so much as direction. Faithfulness is not measured in how quickly one matures, but in the consistency that one matures. In other words, the fact that this grace and knowledge leads toward Jesus Christ is what matters. Remaining stationary is only a little better than apostatizing. Christians move onward toward their Lord and Savior. The speed of that march is of little consequence when compared to consistent trajectory.
In two short commands, Peter has summarized his letter. Believers must remain vigilant and keep a constant watch for prowling wolves and enticing lies. Meanwhile, we are also called to continue spiritual growth by maturing in the grace and knowledge that our Lord and Savior has granted to us. We belong to Him by His grace alone. We know Him because He revealed Himself to us. All the credit and glory goes to Him, alone.
Summary of Peter’s Purpose (v. 18b)
“To Him be the glory both now and to the day of eternity.”
Of all the doxologies found in the New Testament, this is the only one where the second person of the trinity is undoubtedly in view. That is quite a statement considering that this was penned by Simeon Peter (1:1), a Jew who knows the full implications of Is. 42:8: "I am Yhwh, that is My name; I will not give My glory to another, nor My praise to graven images.” There is no doubt that Peter’s confession remains unchanged: Thou art the Christ! The Son of the living God!
It is noteworthy that Peter makes a distinction between his current day (now) and the future day of Christ’s return (day of eternity). He acknowledges the glory of Christ and desires the glory of Christ in the present day, before Christ’s kingdom, as well as in the future after Christ establishes His kingdom. Many people today cling to the idea that Christ is already reigning and ruling His kingdom based on the idea of Christ’s glory and sovereignty. The fact that His kingdom is future (day of eternity) does nothing to rob Christ of glory now. As he offers praise and worship to Christ, Peter simultaneously sends a final dart in the eye of the false teachers.
When considered as a whole, 2 Peter is a simple book. He commands his readers to be holy (ch. 1), warns them of evil men that will hinder that holy calling (ch. 2), and encourages them that Christ will prevail, and the wicked will perish (ch. 3). If 2 Peter is such a simple book, why then is there so much controversy over this material?
Every single one of Peter’s arguments is predicated on the fact that his message is unchanged from the Old Testament prophets and the New Testament apostles. He makes it clear that there is one faith, the same faith as us (1:1). Evangelicalism exists only because it allows for numerous faiths. Evangelicalism must never be confused with Christianity. Christianity is the one true faith preached by Paul and Peter as foreseen by the prophets. Evangelicalism is the umbrella that covers all the various apostate offshoot groups that base their teachings on some Scriptures, but at some point or another depart from the same faith taught by Peter. Peter is very clear. Those who twist the Scriptures do so to their own destruction. Stay away from them. With such a warning, it is of little wonder that this book comes under so much fire from the academy and is avoided by the laity. Yet, because of the warnings contained here, this is the very book that must be studied, preached, and applied. May the Church of Jesus Christ repent of her apathy and love of lies. May she arise holy and blameless. May she forget the taste of false teaching and long only for the pure milk of the word. May our Lord and Savior Jesus Christ find her faithful when He returns.
Soli Deo Gloria!
 Peter Davids, The Letters of 2 Peter and Jude, The Pillar New Testament Commentary (Nottingham, England: Apollos, 2006), p 309-10.  R. C. H. Lenski, The Interpretation of The Epistles of St. Peter, St. John and St. Jude (Minneapolis, MN: Augsburg Publishing House, 1966), p. 357.  Davids, p. 314-17.  Lenski, p. 401.  D. Edmond Hiebert, Second Peter and Jude: An Expositional Commentary (Greensville, South Carolina: BJU Press, 1989), p. 177-9.  Davids, p. 315.