Peter's nickname is "The Rock," but we're guessing it wasn't because he walked around Galilee hitting non-believers over the head with folding chairs. Let's take a look, though, just to be sure.
The writers of the Gospel of John don't go into it, but Matthew explains that Peter gets his nickname because he's "the rock" on which Jesus will build his church (16:18). John's gospel thinks he's a pretty standup guy, too:
We're definitely supposed to like Peter and see him as a bona fide leader. Someone worthy of heading the church once Jesus dies. The first Pope. The keeper of the keys at the Pearly Gates in Heaven. Yep, that's the same guy.
But Peter isn't perfect. And when he messes up, he messes up—big time.
On the night he's arrested, Jesus predicts, "before the cock crows, [Peter] will have denied [him] three times" (13:38). Sure enough, within a few hours, Peter is shrugging his shoulders and saying, "Jesus who?" to some hostile questioners.
Why does Peter deny knowing Jesus? Maybe it's because he's scared. And can we really blame him? His Lord and Savior has been arrested on capital charges and Judea is not known for having a long and thorough appeals process. He's probably right to think that if he goes to bat for Jesus, he'll be next on the religious authorities' hit list.
The Gospel of John throws in a nice little post-resurrection scene with Jesus, in which Peter gets to redeem himself. As they're sitting on the beach, Jesus asks Peter three times if he loves him. "Peter felt hurt" (21:17) that Jesus would question him, but, hey, it had only been a week since he refused to admit he knew Jesus.
The three professions of love wipe away the previous three denials, and Jesus asks Peter to take care of his sheep. All is forgiven and Peter can take his rightful place as caretaker of the church. Order is restored. What's the point? Probably that even if you stray, there's always a chance for redemption. Prodigal son, anyone?
The end of John's Gospel also hints at the fact that Peter is going to be put to death for continuing to stay faithful to Jesus. According to Christian tradition, about 30 years after Jesus died, Peter was also sentenced to death by crucifixion. The only thing Peter asked was that he be crucified upside down because he wasn't worthy to be put to death in the same way as Jesus. Loyal 'til the end.