Jesus was rejected in his hometown (6:1-6) but continued his teaching, called disciples, gave them authority to heal (6:7-13). He fed 5,000, walked on water, and healed the sick (7:30-56). Although some of the religious folks were offended by his ways, some foreigners accepted him readily (7:1-37). He fed another 4,000, which did not impress everybody, and cured a blind man (8:1-26).
When he asked Peter, "Who do you say I am?", Peter replied, "You are the Messiah" (8:27-30).
Then he began to teach them that the Son of Man must endure great suffering and be rejected by the elders and chief priests and scribes, be put to death and rise up three days later (8:31).
The title of Son of Man may be an allusion to the vision of one like a human being coming with the clouds of heaven, the one who was to be given dominion over all nations forever, (Daniel 7:13-14).
Early Christians had to reconcile the suffering and death of this man with the acceptance of him as Messiah. How could one chosen by God suffer the kind of death that he did? Mark's gospel helped them with this theological problem. Jesus said the suffering was inevitable and part of God's plan. (See The Gospel According to Mark, by Morna Hooker, for more on this).
At this point he spoke plainly--not in parables. He intended for them to get it. Peter grasped the message but did not approve of it. Jesus rebuked him strongly (8:32-33).
How hard would it have been for them to continue to follow Jesus after they had heard this news?
Some Christians throughout history have read into this passage a necessity for all Christians to suffer. Was Jesus telling Peter what the Messiah had to go through, or was he preparing him for what all Christians would have to bear? In either interpretation, we can remember throughout the Scripture, God has stood by those who are suffering.