Jesus has shown many signs to the disciples, leading them to realize who He is. Peter has just said confirmed his faith by declaring, Jesus, “You are the Messiah!”
Now, everything pivots in the storyline as Jesus begins to teach them about His upcoming suffering and death and what it will mean to follow Him.
Jesus takes the confession of Peter, and in essence says, "You are correct, I am the Messiah. Now, let me tell you what will happen to Me and to those who will follow Me." The challenge is real and it is one we must consider.