Read Luke 22:31-34 again.
“May sift you as wheat.” Grain was always shaken in some kind of fan or sieve. The grain would remain in the sieve, and the chaff would be blown off. Chaff was usually burned, and this word sometimes meant dried grass or hay. Chaff is used as a figure of wickedness (Psalm 1:4; Matthew 3:12). False doctrines are also called chaff (Jeremiah 23:28), or “chopped straw.”
The destruction of the wicked, and their powerlessness, are often compared to the carrying away of chaff by the wind (Isaiah 17:13; Hosea 13:3; Zephaniah 2:2).
So Christ says that Satan desired to try Peter; to place trials and temptations before him; to trouble him; to see whether anything of faith would remain, or whether all would not be found to be chaff and only a false profession of faith.\
What I am saying is that Satan sought to destroy Peter and the apostles by trying and testing them. But, Jesus said, “I have prayed for thee.”
“But I have prayed for thee.” It is through the prayers of Christ that we never fall from the faith. Because of this we ought to encourage one another.
Peter’s faith did fail, but not completely. He did fall, but not entirely, nor did he forsake his Master or His cause as Judas did.
“When thou art converted.” That is, when you are restored to me and my cause is established then “strengthen thy brethren.”
Think of this, Peter continued for a while, then denied Christ three times, and all the rest of the disciples forsook Him and fled, because they feared losing their lives.
Peter would become more cautious than ever through this trial; and stronger in the faith, which enabled him to strengthen the feeble minded, and support the weak. The end result is that God used him mightily to write I and II Peter the persecuted Christians, and it shows how well he was qualified for this important work.
“That thy faith fail not.” The word faith seems to be used in the sense of attachment to Christ, and the words fail not mean fail altogether.
It is true that Peter failed; and that he did not have the confidence and reliance in Jesus he had before, but the prayer of Jesus was that he might not fall altogether from the faith.