Matthew 16:21

Matthew 16:21 From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.
Jesus does not want us to misunderstand what he is saying; you can see by the many examples of Jesus explaining to things to his disciples primarily and once in a while to the general public. The disciples were a chosen group, but they were still volunteers. Obviously they believed that Jesus was worth leaving what they had known.
Peter's acknowledgement to Jesus that he believed him to be; "Thou art the Christ, the Son of the living God." shows an understanding of what Peter had received from the Father and believed, and now Jesus was going to help fill in some of the missing details of that revelation. Well...the missing details were not as well received compared to the revelation of the Father.

What's the difference you ask? Well I'm glad you did ask.

What Peter received of the Father required nothing of him, it was knowledge, it was a revelation; but as soon as the details of that revelation effected Peter and his life, now comes the rub. This is always the reaction of our human nature to the details of how God effects out life. It is something to be aware of but not afraid of because God has also provided the solution to the problem we are experiencing, Christ himself.
After Jesus' resurrection Peter was all about Christ effecting his life, so much so that God entrusted Peter with the beginning leadership of the church.
Keep reading my friend! Read and receive knowledge, and as you can see Jesus showing you the details that are to effect your life, fear not, Jesus will see you through.