What does John 21:21 mean?

Peter is seeing John (John 21:20), who mentions himself in roundabout ways (John 13:23; 19:26; 21:24). Peter, John, and James were Jesus' closest followers (Matthew 17:1; Mark 14:33; Luke 8:51). When Jesus made a prediction about Peter's eventual fate (John 21:18–19), Peter would have naturally been curious about what would happen to John.

Jesus' response (John 21:22) is not a rebuke, or even a criticism. It's simply a rhetorical question, meaning "what difference would that make in your faith? Just follow Me!"

One reaction to this comment is ironic. Jesus is neglecting to make a prediction, but some early Christians will misinterpret it as a prophecy, anyway. In the time between Jesus making the statement and John's writing of this account, rumors will spread that John would not die until Jesus returned. John will feel the need to refute that mistake directly (John 21:23).