What does John 21:18 mean?
Peter denied Jesus three times (John 13:35–38; Luke 22:61–62). Three times, Jesus made Peter affirm his love and faith (John 21:15–17). Jesus called Peter after a miraculous provision of fish (Luke 5:5–11). Having performed a similar miracle yet again (John 21:4–8), Jesus is about to restore Peter's calling (John 21:19).First, Jesus makes a prediction which is both dire and comforting. The description of being dressed and led, with outstretched hands, is a poetic foreshadowing of crucifixion. The following verse confirms this. Church tradition indicates Peter was crucified and asked to be hung upside down out of respect for Christ's similar death. While this seems like a terrible fate, it implies something Peter would find encouraging: his faith would never fail him again. That doesn't mean Peter would be perfect (Galatians 2:11–14). It does mean his passionate desire to serve Jesus would define the rest of his life.
John 21:15–19 is a poignant moment between Peter and the resurrected Jesus. Early in his ministry, Jesus inspired a faithful response from Peter, by miraculously providing fish (Luke 5:5–8). He then called Peter to become a disciple (Luke 5:10–11). Not many days before this breakfast on the beach, Peter had stood near a fire and denied even knowing Christ three times (John 18:25–27). Now, in front of another fire, Jesus asks Peter to affirm his faithful love three times. Peter grasps the symbolism. Jesus affirms Peter's calling to serve Him and predicts the matter of Peter's death.
After Jesus' resurrection, a group of disciples goes fishing while they wait in Galilee. From a distance, a figure appears and tells the men to cast again. Despite a fruitless night, they obey, and the net is immediately overflowing with fish. This confirms that the man on the shore is Jesus. As the disciples eat a meal with Christ, He presses Peter to reaffirm His faith. Jesus then predicts the nature of Peter's death; He does not do so for John. The gospel of John ends with an affirmation of its source, and a reminder that no single book could contain every detail of Jesus' ministry.