This is the disciple who testifieth — Being still alive after he had wrote. And we know that his testimony is true — The Church added these words to St. John's, Gospel, as Tertius did those to St. Paul's Epistle to the Romans, Romans 16:22.
Related Commentaries of John 21:24
John 21:24
24 This is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true.