But I have prayed for thee, that thy faith fail not: and when thou art converted, strengthen thy brethren. 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.