"By using the word must, Jesus is indicating that he is planning to die—that he is doing it voluntarily. He is not merely predicting it will happen. This is probably what offends Peter the most. It is one thing for Jesus to say, 'I will fight and will be defeated,' and another to say, 'This is why I came; I intend to die!' That is totally inexplicable to Peter…Here Jesus is saying, 'Yes, I’m the Messiah, the King, but I came not to live but to die. I’m not here to take power but to lose it; I’m here not to rule but to serve. And that’s how I’m going to defeat evil and put everything right…' The word must modifies and controls the whole sentence, and that means that everything in this list is a necessity. Jesus must suffer, must be rejected, must be killed, must be resurrected.
Peter found the necessity of Jesus’ death difficult. Why was this so difficult for him—and for us—to accept?"
-Timothy Keller, JESUS THE KING-