Q. According to Luke, at the Last Supper Jesus said to Simon Peter, “Satan has asked to sift all of you as wheat. But I have prayed for you, Simon, that your faith may not fail.” What does “all of you” mean in this context? That Satan wanted “all of Simon,” or that he wanted to sift all twelve of the apostles? If the answer is that Satan wanted to sift all twelve of the apostles, then why did Jesus tell only Simon that he had prayed that his faith would not fail? Why didn’t Jesus pray for all twelve of his disciples?
“All of you” means that Satan wants to sift all twelve of the apostles. The pronoun “you” is plural in the Greek, and the NIV, which you are quoting from, is using the expression “all of you” to reflect that. Other translations say “you apostles,” “you disciples,” or “you men.”
When Jesus addresses Simon specifically, the pronoun becomes singular in Greek. I think Jesus recognized the bravado that Peter in particular was likely displaying as the disciples, in response to Jesus’ warning that one of them would betray him, instead argued about which one of them was the greatest. Sure enough, Peter then boasted that he would never abandon Jesus, and Jesus had to tell him that he was actually going to deny him three times.
But Jesus had Peter in mind all along for a leadership role in the community of his followers after his death, and “strengthen your brothers” is a call to step into that role even after the denial. (It’s like “feed my sheep” at the end of the gospel of John.) As for why Jesus didn’t say that he was also praying that the faith of the other disciples wouldn’t fail, perhaps Peter was at the greatest risk because he was the most insistent that his faith would never fail.