The author of John lays out his purpose near the end of his narrative.
John 20:30-31 – Jesus did many other miraculous signs in the presence of his disciples, which are not recorded in this book. But these are written that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name.
This is an evangelical text. It is an argument laid out in both theological and narrative forms designed to persuade the reader to believe in Jesus of Nazareth.
The date of the writing of the text is generally agreed upon to be between 85 – 120 AD. The earliest fragment of John (and of the New Testament) has been dated probably between 117AD-138AD. An earlier date would lend probability to the claim that this gospel was written by an actual eyewitness and disciple of Jesus identified as “The disciple Jesus loved…” in the text, while a later date would rule this out, though not necessarily rule out the possibility that the account was based on John’s recollections and teachings.
As noted, the text itself does not identify the author directly, choosing the descriptive phrase “The disciple whom Jesus loved…” instead. However:
In the titles, tables of contents, signatures, which are usually added to the text of the separate Gospels, John is in every case and without the faintest indication of doubt named as the author of this Gospel. The earliest of the extant manuscripts, it is true, do not date back beyond the middle of the fourth century, but the perfect unanimity of all the codices proves to every critic that the prototypes of these manuscripts, at a much earlier date, must have contained the same indications of authorship. Similar is the testimony of the Gospel translations, of which the Syrian, Coptic, and Old Latin extend back in their earliest forms to the second century.
John’s authorship of the gospel was accepted until the late 18th century, which doesn’t prove the matter one way or the other, but of greater weight to me is the testimony of Irenaeus. Irenaeus was the disciple of Polycarp who was the disciple of John, and Irenaeus attributes the gospel to the apostle.
So it seems reasonable to accept John as the author of the gospel that traditionally bears his name. And if he is not the direct author, it seems reasonable to assume that his disciples recorded his teachings, testimony, and theology based off the oral accounts he had instructed them in.
There are other theories out there, including some that view the gospel of John as a living document undergoing revision as the theological climate changed and heresies had to be addressed, while others make the claim that the gospel of John was written as a direct counterargument to the proto-gnostic gospel of Thomas.
That said, John does stand apart from the last three gospels we’ve looked at. It’s far more theological than Matthew, Mark, and Luke. The Synoptic gospels are most historical narratives with the author relating events, while John treats the events he records as framing devices for the teachings of Jesus. The divinity of Jesus is more clearly emphasized in John than in the other gospels. And the events related by John differ almost entirely from those recorded in the Synoptics. John places far more emphasis on Jesus’ work in Judea and Jerusalem. The events may be different simply because the author is familiar with the other gospels circulating and decides to tell different stories in his effort to evangelize.
Nevertheless because of these differences, questions remain over the historicity of John.
The Synoptics say that Jesus’ Last Supper was the Passover meal—held that year on a Thursday evening (Jewish holidays begin at sunset)—and they would have us believe that the Sanhedrin, the high court, gathered at the beginning of a major holiday to interrogate Jesus and hand him over to the Romans. John, in contrast, has Jesus handed over for crucifixion on “the day of Preparation of Passover week, about the sixth hour.” According to John, the Last Supper is not a Passover meal (because the holiday that year did not start until Friday evening), and Jesus is crucified and buried before Passover begins. In John’s account Jesus becomes the Passover sacrificial lamb, which was offered the afternoon before the Passover holiday. Some scholars suggest that John may be more historical regarding the crucifixion than the other three Gospels
…all the objections brought forward, John is in agreement with the Synoptists as to the date of the Last Supper. It occurred on Thursday, the thirteenth day of Nisan, and the Crucifixion took place on Friday, the fourteenth. The fact that according to John, Christ held the Supper with His Apostles on Thursday, while, according to the Synoptists, the Jews ate the paschal lamb on Friday, is not irreconcilable with the above statement. The most probable solution of the question lies in the legitimate and widespread custom, according to which, when the fifteenth of Nisan fell on the Sabbath, as it did in the year of the Crucifixion, the paschal lamb was killed in the evening hours of the thirteenth of Nisan and the paschal feast celebrated on this or the following evening, to avoid all infringement of the strict sabbatic rest.
Until additional evidence comes along, I will give the book the benefit of the doubt. If you know of additional arguments for or against John’s authorship and historicity, feel free to link to them in the comments.