Who do you say that I am? is a question preachers normally like to use for a sermon. I think it is a good challenge for us to reexamine the titles for Jesus that we use so casually.
Lord, Messiah, King, Savior
What do we mean by any of them?
It is important to know someone's identity and I think Jesus was assessing the disciples understanding in the passage from Mark 8. He had just been reminding them that they needn't be concerned about bread when they misinterpreted his statement to avoid the yeast of the pharisees. And Mark's rapid-paced narration will soon have Peter, James and John seeing the prophets on the mountain top with Jesus, as if to show that he isn't them but is ONE of them.
Putting things in historical context it appears that the early christian understanding of Jesus was greatly influenced by Qumran's expectations that a mashiah would be a future anointed leader and their expectation of multiple mashiahs of Aaron and of Israel became included in the concept of the Messiah. Of course Paul's early writings influenced all the later understandings too and so we get Mark's Peter saying, "You are the Messiah." (My words don't do justice to the scholarship out there. See the H.C. Bible Dictionary for a nice summary.)
Jesus kinda corrects Peter's assumption maybe because he knew Peter was using the title as lightly as we do, without really understanding what it means. Jesus instead speaks of the Son of Man as his title and predicts his own suffering in that role. Again, historic use of "Son of Man" in Daniel is for an end-time judgment by a heavenly figure. This description begins to fit with one who comes to suffer before returning from death and the eventual expectation that he will come again to judge humankind.
I like that all these descriptions challenge me to think about the names I use for Jesus. All the predictions of suffering are not to be taken lightly just because we are familiar with the story. Jesus tries to share his 'upside-down kingdom' (Kraybill) concept with the disciples by speaking about the cross before it became common to use the term by its association with Christianity. We need to hear that "any who want to be my followers need to deny themselves and be executed". Then we might ask ourselves what would it mean to be executed? Is it a literal statement or metaphorical?
This is the challenge for Christians and the challenge of this passage. How do we 'lose' our lives for Jesus' sake and for the sake of the good news? What in us needs to be denied?