From Mark 8.27 till the end of the chapter, Jesus and his disciples are on a road trip up to the villages of Caesarea Philippi. Along the way, the topic of the identity of Jesus comes up and so Jesus asks his disciples, Who do people say that I am? (8.27) They respond: John the Baptist; and others, Elijah; and still others, one of the prophets (v. 28). But Peter is bold and believing, and he comes forward saying: You are the Messiah! (v. 29).
Jesus begins to tell them that the Son of Man must undergo great suffering, and be rejected by the elders, the chief priests, and the scribes, and be killed, and after three day rise again (v. 31). This evidently did not mesh well with Peter's conception of the role of the Messiah, because he takes Jesus aside and begins to rebuke him (v. 32). But Jesus responds with rebuke and tells Peter, [Y]ou are setting your mind not on divine things but on human things (v. 33).
Then he turns to his disciples and to the crowd and says: If any want to become my followers, let them deny themselves and take up their cross and follow me. For those who want to save their life will lose it, and those who lose their life for my sake, and for the sake of the gospel, will save it (vv. 34-5).
Jesus many times throughout the gospel of Mark insists that persons not tell anyone that he is the Messiah. When Peter affirms that he is the Messiah, Jesus sternly ordered them not to tell anyone about him (v. 30). Why might that be? It is probably because they did not know what the Messiah was supposed to do, they did not know God's actual plan for the Messiah and the salvation he would accomplish. In fact they actively reject that plan and that truth when Jesus tells them quite openly (v. 32) that the Messiah is supposed to suffer, be rejected, and die.
God's plan was otherwise. The Messiah must die because he must give his life as a ransom for many (10.45). And it will be expected likewise of the Messiah's disciples that they give their lives for his sake and for the sake of his gospel. Christianity is demanding; following Jesus is hardly easy, but is rather a death every day (cf. 1 Cor 15.31). It shouldn't surprise us that the spiritual life is painful and difficult; it took pain and difficulty to make it possible for us in the first place. We can have a spiritual life -- a life in the Spirit -- only through the grace and faithfulness of Jesus Christ, who freed us from the curse and death which kept us separated from God.
At the same time, if we do not seek to die along with the Messiah in order that we may enjoy life, the result is rejection: Those who are ashamed of me and of my words in this adulterous and sinful generation, of them the Son of Man will also be ashamed when he comes in the glory of his Father with the holy angels (8.38). If we insist on abiding in a life of sin, then the result will be that we are excluded from the kingdom; but if we suffer alongside the Suffering Servant, the reward is life eternal and fellowship with God.