Jesus accepted Peter's profession of faith, which acknowledged him to be the Messiah, by announcing the imminent Passion of the Son of Man. He unveiled the authentic content of his messianic kingship both in the transcendent identity of the Son of Man "who came down from heaven", and in his redemptive mission as the suffering Servant: "The Son of Man came not to be served but to serve, and to give his life as a ransom for many." Hence the true meaning of his kingship is revealed only when he is raised high on the cross. Only after his Resurrection will Peter be able to proclaim Jesus' messianic kingship to the People of God: "Let all the house of Israel therefore know assuredly that God has made him both Lord and Christ, this Jesus whom you crucified."
Source: Catechism of the Catholic Church (Vatican.va)