They are not aware of what being the Messiah means. Not just victory. Not just winning over oppressors. And it's time for those closest to Jesus to begin to learn this.
Jesus begins to teach them what is going to happen--not just the healings and feedings and water-walking, but also suffering, rejection, and even death.
Peter doesn't like this kind of talk and tries to persuade Jesus to back off some. Jesus is adamant.
Jesus speaks not only to the disciples but to the crowds, "If you want to follow with me, deny yourself, take up your cross, and follow me."
He's talking to us.