From: Mark 8:27-33
Peter's Profession of Faith
 And Jesus went on with His disciples, to the villages of Caesarea Philippi; and on the way He asked His disciples, "Who do men say that I am?"  And they told Him, "John the Baptist; and others say, Elijah; and others one of the prophets."  And He asked them, "But who do you say I am?" Peter answered Him, "You are the Christ."  And He charged them to tell no one about Him.
Jesus Foretells His Passion and Resurrection. Christian Renunciation
 And He began to teach them that the Son of Man must suffer many things, and be rejected by the elders and the chief priests and the scribes, and be killed, and after three days rise again.  And He said this plainly. And Peter took Him, and began to rebuke Him.  But turning and seeing His disciples, He rebuked Peter, and said, "Get behind Me, Satan! For you are not on the side of God, but of men."
29. Peter's profession of faith is reported here in a shorter form than in Matthew 16:18-19. Peter seems to go no further than say that Jesus is the Christ, the Messiah. Eusebius of Caesarea, in the fourth century, explains the Evangelist's reserve by the fact that he was the interpreter of St. Peter, who omitted from his preaching anything which might appear to be self-praise. The Holy Spirit, when inspiring St. Mark, wanted the Gospel to reflect the preaching of the prince of the Apostles, leaving it to other evangelists to fill out certain important details to do with the episode of the confession of Peter.
The sketchiness of the narrative still show Peter's role quite clearly: he is the first to come forward affirming the messiahship of Jesus. Our Lord's question, "But who do you say that I am?", shows what Jesus is asking the Apostles for--not an opinion, more or less favorable, but firm faith. It is St. Peter who expresses this faith (cf. note on Matthew 16:13-20).
31-33. This is the first occasion when Jesus tells His disciples about the sufferings and death He must undergo. He does it twice more, later on (cf. Mark 9:31 and 10:32). The Apostles are surprised, because they cannot and do not want to understand why the Master should have to suffer and die, much less that He should be so treated "by the elders and the chief priests and the scribes." But Peter, with his usual spontaneity, immediately begins to protest. And Jesus replies to him using the same words as He addressed the devil when he tempted Him (cf. Matthew 4:10); He wants to affirm, once again, that His mission is spiritual, not earthly, and that therefore it cannot be understood by using mere human criteria: it is governed by God's designs, which were that Jesus should redeem us through His passion and death. So too, for a Christian, suffering, united with Christ, is also a means of salvation.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.