The Gospel of Mark was written to help us understand who Jesus is, what he came to do, and how we should respond to him. For more information about the structure and outline of this Gospel, see the notes for that section in the commentary on Mark 1:1–13.
The disciples have just learned who Jesus is (Mark 8:29). Now, they find out what he came to do (Mark 8:31) and they also learn what it will mean for them to follow Jesus.
Mark 8:31 – 9:1 is arranged in a very simple manner:
Mark 8:31–33: What has Jesus come to do as the Christ?
Mark 8:31 – 9:1: What does Jesus want from those who follow him?
31 And he began to teach them that the Son of Man must suffer many things and be rejected by the elders and the chief priests and the scribes and be killed, and after three days rise again.