When Jesus asked Peter, "who do you say I am? Peter replied, thou art the Christ, the Son of the living God. And Jesus answered and said unto him, Blessed art thou, Simon Bar-jona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.
Jesus replied Thou art Peter and upon this rock, I will build my church and the gates of Hell shall not prevail against it.
Jesus was describing "the rock" as Peter's confession that Christ IS the son of the living God- this confession and testimony from Peter was and is the Christian church that Jesus said he would build.
Peter's confession was no different than any of the other 12 disciples, He just happened to ask Peter this question,