Why wasn't the gospel of Christ's death and resurrection part of Peter's confession or the message before Acts 8?

At Caesarea Philippi, near the headwaters of the Jordan, Jesus asked the twelve whom men said He, the Son of Man, was. They replied that some said John the Baptist, some Elijah, and others Jeremiah or one of the prophets. When He asked whom they said He was, Simon Peter answered, 'Thou art the Christ, the Son of the living God.' The context stresses that Peter did not add 'who died for me, was buried, and rose from the dead.' This confession of Jesus as the Christ, the Son of the living God, runs from Christ's earthly ministry on through Acts chapter 8 and even into chapter 9. The reason is that God had not yet revealed the tremendous plan of salvation based upon grace—that Christ died for the sins of the world, was buried, and rose from the dead. Up till then, they were to believe who He was, which is the Gospel of the Kingdom. Les explains this distinction to show why the death, burial, and resurrection gospel was not part of Peter's confession or the message before Acts 8. The passage then returns to Acts 8, where Philip baptizes the eunuch.