13 Now when Jesus came into the parts of Caesarea Philippi, he asked his disciples, saying, “Who do men say that I, the Son of Man, am?”

14 They said, “Some say John the Baptizer, some, Elijah, and others, Jeremiah, or one of the prophets.”

15 He said to them, “But who do you say that I am?”

16 Simon Peter answered, “You are the Christ, the Son of the living God.”

17 Jesus answered him, “Blessed are you, Simon Bar Jonah, for flesh and blood has not revealed this to you, but my Father who is in heaven. 18 I also tell you that you are Peter, and on this rock † I will build my assembly, and the gates of Hades‡ will not prevail against it. 19 I will give to you the keys of the Kingdom of Heaven, and whatever you bind on earth will have been bound in heaven; and whatever you release on earth will have been released in heaven.” 20 Then he commanded the disciples that they should tell no one that he was Jesus the Christ. 21 From that time, Jesus began to show his disciples that he must go to Jerusalem and suffer many things from the elders, chief priests, and scribes, and be killed, and the third day be raised up.

22 Peter took him aside, and began to rebuke him, saying, “Far be it from you, Lord! This will never be done to you.”

23 But he turned, and said to Peter, “Get behind me, Satan! You are a stumbling block to me, for you are not setting your mind on the things of God, but on the things of men.”

Matthew 16:13-23 - World English Bible

Self identity is one of humanity's great mysteries. How do I know who I am? Post-modern philosophy answers that question in a social context. I know my place, my purpose, my sense of self in relationship to others. “I am” is a social construct.

Ancient societies measured the self in the same terms. “Who do you say I am?” was a question of social definition. Jesus asked it, in one sense, to find out what his band of disciples thought of him. The “Son of Man” was the spirit of the Baptist or one of the great prophets lived out among them. Or was he? Simon answered, “You are the Christ, the Son of the living God.” In other words, he was the Messiah. For Simon's answer, Jesus defined the head of the group with a new identity; he was “Peter,” the rock of the disciples' belief, for his faith in Jesus would be the cornerstone of the Church, his faith would open the way to the Kingdom of heaven for others.

Once Peter established the identity of the Jesus for the group (and the identity of the group itself as Church), the Lord could answer the question himself as the One who would suffer, die, then rise from the dead. But, Simon now Peter disputed that understanding. For his objection, Jesus threatened to eject him from the group. Simon Peter had no right to define the identity of the Messiah and, by extension, the self understanding of the disciples. Jesus connected the notion of the Christ to the cross; those who followed him would go to the cross, as well.

How do you define yourself as a Christian? Where is the cross in your self understanding as a disciple?