"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" (John 21:24, RSV).
John 21:24 is the final postscript to the last chapter of the Gospel of John which in itself is regarded as a kind of appendix to the previous 20 chapters. In the last-but-one verse of chapter 21 there is an identification of the Beloved Disciple as the one upon whose testimony the gospel rests. But who is or were the author(s) of the last two verses of chapter 21? The presence of the words "we" and "his" cast a shadow of doubt not only on the authorship of John 21:24 but perhaps even upon the whole of John's Gospel.
It need not be assumed that what is stated in John 21:24 necessarily implies that the Beloved Disciple wrote the gospel itself, or even whatever traditional material the evangelist used, any more than the phrase "Pilate also wrote a title" means that Pilate himself wrote the title nailed to the cross. The beloved disciple need have done no more than "caused to be written" or "dictated to a secretary".
The transition to the first person plural in the second part of the verse probably indicates, as the great majority of scholars would hold, that the elders of the Assembly (Church) at Ephesus add their testimony to the work of the author as he makes available to the world the profound witness of the Beloved Disciple.
In my opinion, then, the writer of chapters 20-21 of the gospel is the Beloved Disciple whom I also believe is John the Apostle. Chapter 21 seems to me to have been an afterthought by the same author which would explain the 'hiccough' between 20 and 21. It would then seem as though a scribe, or scribes, whilst copying out the complete gospel, decided to add their testimony in what is now 21:24. I do not believe that the addition here in any way affects the authenticity of the gospel as a whole although it is possible that additions and subtractions could have been made to and from the original by Christians with the intent of clarifying doctrines and perhaps even refuting heretics.
John 21:24 also reveals to us how the scribes who inserted their testimony viewed John's overall purpose in writing the gospel: that is, to bear witness of the truth. This would fit in with the internal evidence of the gospel as a whole which suggests that John's purpose is not so much as to relate history (as is the Synoptists' intention) but to bear testimony that Yah'shua (Jesus) is the Son of Yahweh, the Messiah of Israel come in the flesh. This follows the pattern of John's testimony in his first epistle.
There is some useful external evidence to suggest that John the Apostle is the author of the gospel and that it is him the Ephesian believers are bearing witness of in 21:24. One of the earliest surviving pieces of evidence for authorship is in the writings of Irenæus. He had lived in the east around A.D.130. He moved to the west and was consecrated Bishop of Lyons around A.D.178 and died around A.D.200. He reported that after the other gospels had been written that John, the disciple of Yah'shua (Jesus), who had even rested on His breast, had written a gospel whilst living in Ephesus in Asia. Moreover, Irenæus said that when he was a boy in lower Asia, he had seen Polycarp (ca. A.D.69-155), the Bishop of Smyrna, and that he could remember that Polycarp had talked about John and the others who had seen Yah'shua (Jesus). There is thus a direct link between Irenæus, Polycarp, and John who, Irenæus says, was the author of the gospel.
Other Christian writers of this period and later held the same opinion about the authorship of the gospel, with two exceptions: some heretical teachers, called by their opponents the Alogi (ca. A.D.170) said that the author was Cerinthus, a heretic who lived around A.D.100, and Ephrem Syrus (ca. A.D.306-73) said it was written at Antioch.
Although the testimony of Irenæus appears very strong indeed, it has been disputed, because to many scholars it seems impossible that the Fourth Gospel could have been written by John, the son of Zebedee. Although it is possible that Irenæus's memory could have been faulty (as he was, after all, recalling events that had taken place some 45 years previous to his writing Against Heresies in A.D.180) it is not sufficient to criticise John on the basis of his humble, uneducated parentage. History has too many examples of illiterate peoples eventually attaining to the heights of literary composition as the result of self-education. We must remember that John also enjoyed the Ruach haQodesh (Holy Spirit) as a tutor.
The internal evidence of the authorship of John's Gospel comes out no more clearly than the external. It has been suggested that the Beloved Disciple was an imaginary figure, the ideal disciple of Yah'shua (Jesus). Yet 21:24 would seemingly clearly refute this claim. In the final analysis, the identity of the author must remain within the realm of personal conviction: and as for myself, I am satisfied in my own mind that the Beloved Disciple was one and the same with John, the son of Zebedee, and John the Elder or Presbyter. History may never be able to establish the truth of the matter but the testimony of the gospel will always remain. Perhaps, when the scribes of Ephesus added their testimony, they had in mind the Old Testament injunction: "In the mouths of two or three witnesses is the truth established." If this is so, then John's gospel is established as true; the Synoptists indirectly testified of each other -- now the Ephesian believers testify of John's account, the last of the witnesses of the life and teachings od Christ.