By the "fourth gospel," I mean the gospel commonly attributed to an "apostle John."
However, is there actually an internal evidence that someone named John wrote the fourth gospel, as is commonly believed (and, it is commonly believed, else why would most people call it the Gospel of John)?
The primary source for laying out a case for the author's identity is John 21.24-25, the final two verses of the book. Here we are given a brief glimpse at the book's origin: it seems to have been written by or based on the testimony of an individual we identify as the Beloved Disciple ('This is the disciple who is bearing witness about these things', 'I suppose that...') for the benefit of his Christian community ('we know that his testimony is true').
Tradition has long said this Beloved Disciple was John, the son of Zebadiah. Some of the earliest surviving manuscripts of the Gospel (P66 and P75) contain the title 'According to John'. This was also the opinion of several early Church writers, including Dionysius of Alexandria, Origen, Eusebius, Clement of Alexandria, Irenaeus, and the Muratorian fragement.
It should be clear: without the Gospel directly naming its author, all evidence is circumstantial at best. The most we can do is formulate an opinion based around possible interpretations of this circumstantial evidence. So my answer here is not meant to be a definitive answer to the question, only one possible answer.
We should clarify a few cases of who the Beloved Disciple cannot be. Unless we assume a later editorial process took place to purposely obscure his identity, the Beloved Disciple cannot be:
In the fourth Gospel, John, the son of Zebadiah, is mentioned only once. This single reference (John 21.2) is indirect – he is not even mentioned by name, we infer his presence in the epithet 'the sons of Zebedee'. This is the only passage where John is mentioned (even indirectly) in proximity to the Beloved Disciple (who is found in 21.7).
While tradition strongly favors equating the two, the text itself makes no effort to do so, and even implies against it. Objectively speaking, the Beloved Disciple could just as well have been Nathanael, the other 'son of Zebedee', or one of the other 'two disciples' mentioned in John 21.2.
A handful of scholars have suggested the Beloved Disciple should be understood as Lazarus. Ben Witherington III makes perhaps the strongest case, but some of his arguments depend on a close corroboration between the fourth Gospel and the Synoptics, which is not what the present question is allowing. I will here lay out some of the internal evidence that, though ultimately circumstantial, may be seen to identify Lazarus as the Beloved Disciple, and thus the author/source of the book's content.
1. 'Love' is one of the major themes of the fourth Gospel, but for the first ten chapters of the book, no individual is singled out as the recipient of Jesus' love. None of the disciples are a singular focus when it comes to Jesus 'loving' someone. When we get to chapter 11, we are suddenly introduced to the siblings Lazarus, Martha, and Mary. While verse 11.5 does identify all three as people 'Jesus loved', it is clear that Lazarus is the primary focus of this 'love' (11.3,36).
But just as Lazarus disappears from the narrative (last mentioned in 12.17), the Beloved Disciple suddenly enters the story (first mentioned in 13.23). Again, of all of Jesus' followers, this particular disciple is the only one described as 'loved' by Jesus in a unique way. It is quite possible readers were meant to make this connection, and infer that Lazarus was the Beloved Disciple.
It otherwise becomes an extremely unusual coincidence that chapters 11-13, the place where the book's narrative pivots from signs narrative to passion narrative, would introduce two individuals uniquely identified as the one(s) 'whom Jesus loved'.
2. 'Life' is another one of the major themes of the fourth Gospel. The pre-passion narrative (chapters 1-12) are structured around seven 'signs' that Jesus performs as evidence for his identity as the messiah. The seventh sign, which is the turning point of the book from the signs narrative over to the passion narrative, is the raising of Lazarus from the dead.
3. The Beloved Disciple is specifically portrayed as the first to 'believe' that Jesus has been raised from the dead (20.8-9). This could possibly be because, internal to the narrative, the Beloved Disciple himself had been raised from the dead.
4. The final bit of narration in the Gospel is 21.15-23. After having just been told he will die for Jesus' sake (21.18-19), Peter asks Jesus what would become of the Beloved Disciple (21.20-21). Peter's question and Jesus' response are mistakenly understood by the rest of the group as meaning the Beloved Disciple 'was not to die'; the narrator goes out of his way to stamp out this misconception. But we must ask, why was Peter prompted to ask about the Beloved Disciple's fate after having just learned his own fate involved martyrdom? Why did the other disciples seem to think Jesus' answer meant the Beloved Disciple would never die? This could possibly be because, internal to the narrative, the Beloved Disciple had been raised from the dead once, and the other disciples considered that he would not die again.
Several years ago, I was reading through the Fourth Gospel about every week. During this time, one of the things I noticed was the way in which the author refers to Peter. Matthew, Mark and Luke almost almost refer to him simply as "Peter", the major exception being in the retelling of accounts prior to Peter's meeting with Jesus. On the other hand, the author of the Fourth Gospel refers to him very often as "Simon Peter"--17 times, in fact. He is only referred to as "Peter" about 16 times, and most of these are in close proximity to the use of the term "Simon Peter".
As you will recall from the Fourth Gospel, Jesus gave him this name very early in his association with Jesus.
He first found his own brother Simon and said to him, “We have found the Messiah” (which means Christ). He brought him to Jesus. Jesus looked at him and said, “You are Simon the son of John. You shall be called Cephas” (which means Peter). John 1:41-42 ESV
It seems, then, that those who met Peter after this event would really know him as Peter and not as Simon. In fact, the only people who would know him as "Simon" would be those who were associated with Peter prior to his encounter with Jesus and his reception of a new name.
John, the Son of Zebedee, fits this description, as does James, his brother, and Andrew, Peter's brother. However, Andrew is specifically named in the Fourth Gospel and is not, then, "the disciple whom Jesus loved".
James is ruled out as the author as well, since he dies too early after Jesus' resurrection to have been the author.
bout that time Herod the king laid violent hands on some who belonged to the church. 2 He killed James the brother of John with the sword. Acts 12:1-2 ESV
Thus, we are left with John, the brother of James and the son of Zebedee, as the most likely candidate. He was associated with Peter prior to Peter receiving a new name.
And so also were James and John, sons of Zebedee, who were partners with Simon. And Jesus said to Simon, “Do not be afraid; from now on you will be catching men. Luke 5:10 ESV
So, we are left with these facts:
Thus, the best conclusion seems to be that it was, in fact, John.
First of all, Elaine Pagels points out that the 'beloved disciple' is consistently referred to with the disciple Peter, always such as to be the more worthy disciple. For example, compare Luke 24:12, where Peter ran alone to the sepulchre, with John 20:4, where the two disciples ran to the sepulchre but the beloved disciple outran Peter and came first to the sepulchre, or see John 21:20-22. Pagels says that the author of John was trying subtly to put Peter down, perhaps because he had become too venerated, even worshipped, in the view of the author of this gospel. In that context, the use of the term 'beloved disciple' keeps reminding the intended audience that it is not Peter whom Jesus loved, but this other disciple. And that means the beloved disciple was a literary construct, not a usage that could lead us to the real author of the fourth gospel.
Burton L. Mack, author of Who Wrote the New Testament, says that by the middle of the second century, John's Gospel had become popular in gnostic circles where it was said that Cerinthus, the founder of a gnostic school, had written it. Of course, the Orthodox branch of Christianity could never have accepted an attribution to a leading gnostic, so the Gospel had to be attributed elsewhere if the Gospel was to become an Orthodox scripture. Whether or not Cerinthus was the real author, the attribution to John the disciple must have been quite late in the second century.
In the book there are several references in the third person, the "beloved disciple", which indicates that the author is speaking of himself (the author uses first names when referring to the other apostles, Peter, James, etc.).
"Then Peter, turning about, seeth the disciple Jesus loved Whom Following
(...)
This is the disciple Which testifieth of these things, and wrote these things: and we know que his testimony is true. John 21:20,24 KJV "
