Note 17 at Jn. 21:23: John is writing this gospel around the end of the first century (see date of writing--Introduction to John, p. xxi) and he includes this information to correct a misunderstanding of Jesus' statement which he had probably heard many times. Apparently, some people had interpreted Jesus' comments to Peter to mean that John would not die before the second return of Jesus. However, John makes it clear that Jesus didn't say that. Jesus simply stated to Peter that if He willed that John remained alive till He comes again, that should not affect the way Peter follows Him.