Albert Barnes Commentary Luke 22:32

"but I made supplication for thee, that thy faith fail not; and do thou, when once thou hast turned again, establish thy brethren." — Luke 22:32 (ASV)

That your 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 did not have that immediate confidence in Jesus and reliance on him which he had before; 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. It also follows, since 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 prevailed, and he was brought to true repentance.

When you are converted. The word converted means turned, changed, recovered. The meaning is, when you are turned from this sin, when you are recovered from this heinous offence, then use your experience to warn and strengthen those who are in danger of similar sins. A man may be converted or turned from any sin, or any evil course. He is regenerated only once—at the beginning of his Christian life; he may be converted as often as he falls into sin.

Strengthen your brethren. Confirm them, warn them, encourage them. They are also in continual danger 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 those 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.

See Barnes on John 21:18.

Regarding "strengthen": Psalms 51:13; John 21:15–17.