John's Gospel is widely thought to be the least historical and trustworthy of all the gospels, mainly because John shows a great difference in the sequence and type of his narrative from the Synoptic (Matt, Mark, Luke) Gospels. However, on first sight it looks as though John can claim the highest authority, as it claims to have been written by John, brother of James, an eyewitness, and unlike Matthew (the only other gospel with an 'author' who might be an eyewitness) does not depend uon another gospel for its source.
Traditionally, people have believed the Gospel to have been written by John son of Zebedee, who is identified as "the disciple whom Jesus loved" and which seems to be indicated in 21:24; "this is the disciple which testifieth of these things, and wrote these things: and we know that his testimony is true". However, the majority of scholars cast doubts upon this claim.
The earliest references to the apostle John come in Justin Martyr's work, around the middle of the second century. When he writes of John, he says that the Apocalypse (Revelation) is his work (Dial. lxxxi.4) but makes no mention of the Gospel at all, let alone that John wrote it. By 165CE the Gospel was being used by many people, but in the "Homily on the Pasch" of Melito of Sardis, no mention of any author of the gospel is made. It has been suggested that the Valentinian Gnostics - an heretical sect - were the first to identify John as the author of the Gospel, as apostolic authorship for a work they considered as supporting their view would be useful. The earliest non-heretical reference to the authorship of John comes from Irenaeus who said:
"Then John, disciple of the Lord, who also lay on his breast, himself published the gospel, while he was staying at Ephesus in Asia" (Adv. Haer. III.i.1f)
One man of those days appears to have had the noble ambition of gathering together all the sayings and teachings of the original apostles from trustworthy sources so that everyone could know what they really said. That man was Papias, but unfortunately his books have been lost. However, a fragment is preserved in a quotation from Eusebius (HE III.xxxix.1-7):
"If anyone who had been a follower of the Elders came, I enquired about the words of the Elders - what Andrew or Peter said, or what Philip, or what Thomas or James, or what John or Matthew or any other of the disciples of the Lord [said], and the things which Aristion and the elder John, disciples of the Lord, were saying."
Eusebius pointed out that Papias mentions two disciples called John, one dead, and one still living. He identified the first as the author of the Gospel, and the second as the author of Revelation. Jerome (de viris inl. xviii) also gives this quote, but identifies "the elder John" as "the Elder" of 2 and 3 John. We should, however, note that Papias himself makes no mention of the gospel, or of John being the author.
Some scholars have thought that John was martyred alongside his brother James (Acts 12:2), taking their cue from a quotation of Papias by Philip of Side (c. 430CE) that John and James were killed by the Jews. But this does not mean they were martyred together. There is, however, a strong tradition that John was martyred, as borne out, not only by Papias, but by Origen as well. However, as Eusebius (H E III.i.I) mentions John's death at Ephesus, it does not seem as though John could have been martyred at the same time as James.
Now for the internal evidence. There is mention of a "disciple whom Jesus loved" in 13:23, 19:26, 20:2 and 21:7-20. However, it is thought the Beloved Disciple may also be the unnamed disciple in 1:35, 18:15 and 19:35. So we get a picture of one of the earliest disciples, whom Jesus loved especially, who leaned on his breast and who asked him questions the others were too scared to ask. This disciple was present during the Crucifixion, along with Mary and other women, and is said (21:24) to have written the Gospel. This disciple is also the first to believe the resurrection, and is promised long life in 21:22. However:-
Do we even know John the Apostle was the Beloved Disciple? The reason scholars think he was is because he was one of the three disciples in the Synoptics who were present at the raising of Jairus' daughter, the Transfiguration and Gethsemane. However, John's Gospel does not say that there were only three disciples at those events (and does not even include two of them) so we cannot know that this was John. The author also mentions seven disciples, including the sons of Zebedee and two unnamed disciples in 21:2, suggesting that the Beloved Disciple is one of these four in 21:7. As John is so careful not to reveal his identity in the Gospel however, it seems more likely that the Beloved Disciple was one of the two unnamed disciples. As Lindars says: "The identification with John the Apostle is not only unproven, but also distinctly doubtful" (Lindars, B "The Gospel of John" Marshall, Morgan & Scott, 1972 p33).
There have been three other suggestions for an eyewitness author:-
So, we really have no idea who wrote the Gospel, if they were an eyewitness, if they were actually present during any of it - for all we know it could have been written by someone who was just making things up! The identification with John the Apostle is late, and probably there because the gnostics needed apostolic authority for their ideas. We cannot say that John is more or less historical, on the basis of authorship, than the other Gospels, or the apocryphal Gospels.