But after I have risen. He now expresses more clearly -- what I lately hinted -- that the disciples, struck with dread, will resemble for a short time scattered and wandering sheep, but will at length be brought back to the fold. For Christ does not simply say that he will rise again, but promises to be their leader, and takes them for his companions, as if they had never swerved from their allegiance to him; and, to impart to them greater confidence, he mentions the place where they will again meet; as if he had said, "You, who are scattered at Jerusalem, will be again assembled by me in Galilee."

That thy faith fail not - The word "faith," here, seems to be used in the sense of religion, or attachment to Christ, and the words "fail not" mean "utterly fail" or fail altogether - that is, apostatize. It is true that the "courage" of Peter failed; it is true that he had not that immediate confidence in Jesus and reliance on him which he had before had; but the prayer of Jesus was that he might not altogether apostatize from the faith. God heard Jesus "always" John 11:42; it follows, therefore, that every prayer which he ever offered was answered; and it follows, as he asked here for a specific thing, that that thing was granted; and as he prayed that Peter's faith might not utterly fail, so it follows that there was no time in which Peter was not really a pious man. Far as he wandered, and grievously as he sinned, yet he well knew that Jesus was the Messiah. He "did know" the man; and though his fears overcame him and led him to aggravated sin, yet the prayer of Christ was prevalent, and he was brought to true repentance.
When thou art converted - The word "converted" means turned, changed, recovered. The meaning is, when thou art turned from this sin, when thou art recovered from this heinous offence, then use "your" experience to warn and strengthen those who are in danger of like sins. A man may be "converted or turned" from any sin, or any evil course. He is "regenerated" but once - at the beginning of his Christian life; he may be "converted" as often as he falls into sin.
Strengthen thy brethren - Confirm them, warn them, encourage them. They are in continual danger, also, of sinning. Use your experience to warn them of their danger, and to comfort and sustain them in their temptations. And from this we learn:
1. That one design of permitting Christians to fall into sin is to show their own weakness and dependence on God; and,
2. That they who have been overtaken in this manner should make use of their experience to warn and preserve others from the same path.
The two epistles of Peter, and his whole life, show that "he" was attentive to this command of Jesus; and in his death he manifested his deep abhorrence of this act of dreadful guilt in denying his blessed Lord, by requesting to be crucified with his head downward, as unworthy to suffer in the same manner that Christ did. Compare the notes at John 21:18.

I have prayed for thee - From the natural forwardness and impetuosity of thy own spirit, thou wilt be brought into the most imminent danger; but I have supplicated for thee, that thy faith may not utterly fail - εκλειπῃ, from εκ, out, and λειπω, I fail, to fall utterly or entirely off. Peter's faith did fail, but not utterly: he did fall, but he did not fall off, apostatize, or forsake his Master and his cause finally, as Judas did. Every body sees, from Peter's denial of his Lord, that his faith did fail, and his great courage too; and yet they read, in the common translation, that Christ prayed that it might not fail: can they then conceive that our Lord's prayer was heard? The translation which I have given above removes this embarrassment and apparent contradiction. It was certainly Peter's advantage that our Lord did pray for him; but it was not so much for his honor that he should stand in need of such a prayer, beyond all others. Lightfoot.
When thou art converted - Restored to a sense of thy folly and sin, and to me and my cause - establish these thy brethren. All the disciples forsook Jesus and fled, merely through fear of losing their lives; Peter, who continued for a while near him, denied his Master with oaths, and repeated this thrice: our Lord seems to intimate that, after this fall, Peter would become more cautious and circumspect than ever; and that he should become uncommonly strong in the faith, which was the case; and that, notwithstanding the baseness of his past conduct, he should be a proper instrument for strengthening the feeble minded, and supporting the weak. His two epistles to the persecuted Christians show how well he was qualified for this important work.

(11) But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren.
(11) It is through the prayers of Christ that the elect never utterly fall away from the faith: and because of this they should encourage one another on.

But I have prayed for thee,.... Christ prayed for all the apostles; but particularly for Peter, because he was in the greatest danger: whether the prayer Christ refers to was that in John 17:1 in which are many passages relating to the preservation, sanctification, final perseverance and glorification of the apostles, as well as of other saints, as in John 17:9 and so these words might be spoken a little after that prayer was ended, which was about this same time; or whether it was any other, and only mental, and not vocal, is not certain: however, the petition was,
that thy faith fail not; Satan in his temptations strikes principally at the faith of God's people; that being a grace which gives much glory to God, and in the exercise of which believers have much peace, joy, and comfort; both which he envies and grudges; and it is also a shield which keeps off, and quenches his fiery darts, and is a piece of armour he is sadly harassed with, and therefore endeavours all he can to weaken and destroy it, or wrest it out of their hands: but though, through the power of sin, and the force of temptation, it may fail as to some degree of the steadfastness of it, as to the acting and exercise of it, and as to the sense believers may have of it; yet never as to its principle, it being an irrevocable gift of God's grace; a work of his almighty power; a solid and substantial grace, even the substance of things hoped for; an immortal and incorruptible seed, and of which Christ is the author and finisher; and to nothing more is its security owing, than to the prayers of Christ, which are always heard, and to his powerful mediation, and prevalent intercession; Christ is the advocate of his people; he prays that they might have faith, and then he prays, that it may not fail; and it shall not, notwithstanding all the opposition of hell, and earth, unto it:
and when thou art converted, strengthen thy brethren: Peter was now a converted man, and had been for some years; but whereas he would fall by temptation into a very great sin of denying his Lord, and which was attended with such circumstances as made him look like an unconverted, and an unregenerate man; his recovery by the fresh exercise of faith in Christ, and repentance for his sins, is called conversion: and which was not his own act, but owing to the power and efficacy of divine grace; see Jeremiah 31:18. Some versions render it in the imperative, "in time, convert, turn, or return, and strengthen thy brethren"; as the Syriac, Arabic, and Persic versions: as he afterwards did: for whereas all the disciples forsook Christ, and fled, some one way, and some another, Peter, after his recovery, got them together again, and returned with them to Jerusalem; when they with him assembled together, till the third day Christ was risen: he strengthened their faith in the Messiah, and put them upon filling up the place of Judas, by choosing another apostle; and on the day of "Pentecost" preached a most excellent sermon, which as it was made useful for the conversion of three thousand sinners, was, doubtless, a means of confirming the minds of the disciples; and he has left two exceeding useful epistles for the strengthening of his brethren in all ages of time; the design of which is to establish the saints in faith and holiness, that they may not be drawn aside, and fall from the steadfastness of their faith, either by the lusts of the flesh, or by the persecutions of men, or by the error of the wicked.

But I have prayed--have been doing it already.
for thee--as most in danger. (See on Luke 22:61-62.)
fail not--that is, entirely; for partially it did fail.
converted--brought back afresh as a penitent disciple.
strengthen, &c.--that is, make use of thy bitter experience for the fortifying of thy tempted brethren.

But I have prayed for thee - Who wilt be in the greatest danger of all: that thy faith fail not - Altogether: and when thou art returned - From thy flight, strengthen thy brethren - All that are weak in faith; perhaps scandalized at thy fall.