Peter and the Twelve finally understand Jesus to be the Messiah, but they don’t understand rightly what being the Messiah means. Jesus has taught them that the plan all along was for Him to be rejected, killed, and raised again on the third day.
But they also have to learn what it means to follow the Messiah. It is not marching in the triumphant victory parade; it’s carrying the crossbeam of your own crucifixion. Not only must Jesus be rejected and killed and raised again; His disciples have to reject themselves and die to their sinful flesh and be raised to new life in Jesus.
To try to avoid this life of self-denial and dying to sin is to reject Jesus. You cannot save your life, gaining even the whole world in the process, and still keep your soul. And to reinforce the nature of what hangs in the balance of the choice, Peter, James, and John are given an glimpse of Jesus in His glory on the Mount of Transfiguration.
Notes and audio are available here.