Mark has reported a series of miracles--walking on the water, several healings, two feedings of large crowds.
Who can do these things?
Jesus asks his followers, "Who do people think I am?" The disciples give a list of forerunners to the Messiah. Then Jesus asks "Who do you think I am?"
Peter answers for them, "You are the Messiah." Jesus instructs them not to tell anyone.
The crowds are ready to know that the Messiah is coming, but not ready to realize that he is here, among them.