Jesus explained that He would build the church on the Rock, but only after He knew the disciples began hearing revelation from the Father. The Father's revelation was about Christ's identity as Son of God. Once that was made known, Jesus started to reveal His revelation for them -- His need to die on the cross and their need to carry their crosses. Peter received the first revelation from the Father, but resisted the second from the Son. Jesus warned them earlier about knowing Him and calling Him Lord, but not being able to enter the Kingdom because He did not know them. He only knows those who carry their crosses and follow Him. Learn how to be changed into His very image, by carrying your cross and denying yourself. That kind of Christianity is like a house on a rock that cannot fall despite the storms that hell can hurl at us!