From: John 21:20-25
Peter's Primacy (Continuation)
 Peter turned and saw following them the disciple whom Jesus loved, and who had lain close to His breast at the supper and had said, "Lord, who is it that is going to betray You?"  When Peter saw Him, he said to Jesus, "Lord, what about this man?"  Jesus said to him, "If it is My will that he remain until I come, what is that to you? Follow Me!"  The saying spread abroad among the brethren that this disciple was not to die; yet Jesus did not say to him that he was not to die, but, "If it is My will that he remain until I come, what is that to you?"
 This is the disciple who is bearing witness to these things, and who has written these things; and we know that his testimony is true.
 But there are also many other things which Jesus did; were every one of them to be written, I suppose that the world itself could not contain the books that would be written.
20-23. According to St. Irenaeus ("Against Heresies", II, 22, 5; III, 3, 4) St. John outlived all the other Apostles, into the reign of Trajan (98-117 A.D.). Possibly the evangelist wrote these verses to dispel the idea that he would not die. According to the text, Jesus does not reply to Peter's question. The important thing is not to be curious about what the future will bring but to serve the Lord faithfully, keeping to the way He has marked out for one.
24. This is an appeal to the testimony of the disciple "whom Jesus loved" as a guarantee of the veracity of everything contained in the book: everything which this Gospel says should be accepted by its readers as being absolutely true.
Many modern commentators think that verses 24 and 25 were added by disciples of the Apostle, as a conclusion to the Gospel, when it began to be circulated, a short time after St. John completed it. Be that as it may, the fact is that both verses are to be found in all extant manuscripts of the Fourth Gospel.
25. St. John's account, written under the inspiration of the Holy Spirit, has as its purpose the strengthening of our faith in Jesus Christ through reflecting on what our Lord said and did. Like the Fourth Gospel, we shall never be able to capture the full richness and depth of our Lord's personality. "Once one begins to be interested in Christ, one's interest can never cease. There is always something more to be known, to be said--infinitely more. St. John the Evangelist ends his Gospel making this very point (John 21:25). Everything to do with Christ is so rich, there are such depths for us to explore; such light, strength, joy, desire have their source in Him. [...] His coming to the world, His presence in history and culture and [...] His vital relationship with our conscience: everything suggests that it is unseemly, unscientific and irreverent ever to think that we need not and cannot advance further in contemplation of Jesus Christ" ([Pope] Paul VI, "General Audience", 20 February 1974).
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.