A common notion floating around in Pop Culture is that "modern scholarship" has somehow proven the Gospel of John is more or less unhistorical fantasy written by a pseudonymous author.
Here’s the facts: the tradition of the Church, supported by the unbroken line of patristic testimony, as well as internal evidence from the text itself, is that the gospel is rooted in the testimony of the Apostle John, son of Zebedee.
St. Irenaeus tells us (circa 180 A.D.) that the fourth gospel was published by the Apostle John, the teacher of his own mentor Polycarp. Numerous other witnesses in the second and third centuries corroborate this basic witness. In addition, various elements within the gospel strongly suggest John as the author. Most obviously, there is the attestation of the witnesses penning the gospel that it is the testimony of "the disciple whom Jesus loved" (John 21:24)—a disciple to whom no one but John corresponds. The source of the gospel is, quite clearly, a Jew familiar with the conditions of Palestinian Judaism at the time of Christ. He speaks Aramaic and Greek. He knows Jerusalem as it looked before Rome reduced it to rubble in 70 AD. And he gives countless details which, if they are not the testimony of a first-hand eyewitness who was present at the Last Supper, are an absolutely isolated occurrence of novelistic realism nineteen centuries ahead of its time. That he was part of Christ's "inner circle" of Peter, James and John (cf. Galatians 2:9) is even more likely given that he was the disciple at the Last Supper who laid his head on Christ's breast. He can't be Peter, who is distinguished from him in the text, and he can't be James (who died in the early 40s). So it all points to John.
Additionally, the patristic tradition that the gospel was composed in Ephesus also points to John. First, this is the city associated with the Assumption of the Virgin who was commended into his care. Second, the gospel repeatedly answers a sect devoted to John the Baptist with the reply that John “was not the Light” but had only come to “bear witness to the Light” (John 1:8). We know from Acts 18:24 and 19:1-7 that there was such a sect centered in Ephesus. Finally, the sophistication of the gospel fits the fact that the New Testament epistle with the most sophisticated exposition of theology is the epistle to the Ephesians.
Conclusion: all the evidence point to the accuracy of the Church’s tradition that John published his gospel in Ephesus in the second half of the first century.
Some critics, eager to look for cracks in this evidence, will note that the Greek of John’s gospel and epistles is of a different quality than the Greek of John’s Revelation and say, along with Eusebius, that Irenaeus might have had his Johns mixed up between multiple individuals. Others, grasping at straws, may claim that Mark 10:38-39…
But Jesus said to them, "You do not know what you are asking. Are you able to drink the cup that I drink, or to be baptized with the baptism with which I am baptized?" And they said to him, "We are able." And Jesus said to them, "The cup that I drink you will drink; and with the baptism with which I am baptized, you will be baptized.
…implies that both James and John suffered a martyr's death, contradicting John 21:22-23.
But these arguments are weak as well. To be sure, there is a strain of thought dating back to Eusebius that John the apostle and John the “elder” may be two different people. But so what? We know from internal evidence (John 21:24), that the gospel has more than one hand involved in its composition. Given the common use of an amanuensis (a secretary who took dictation) in the New Testament, that shouldn't surprise us. The editors of John make it abundantly clear that they have some sort of hand in the composition of the gospel, but that the gospel is nonetheless rooted in the testimony of the “beloved disciple” whom they know intimately.
This means the discrepancy in writing styles between the Gospel and the Revelation could be due to any number of factors. It may be that John wrote his gospel with the help of another person named John (then, as now, a common name). It may be that he had no amanuensis when he wrote his Revelation (which would explain the different styles and the difference in competence in Greek). None of this disproves the strong evidence that John bar-Zebedee is the source of the testimony in the gospel.
Likewise the attempt to pit Mark 10:38-39 against the testimony of John 21 is what happens when you let all your biblical interpretation be done by people who are on a single-minded mission to show that everything in the Bible is false, untrustworthy, etc. Such determined misreaders wind up forgetting that it's a human book using human language in their zeal to prove it's not God's book. So the critic sets himself the absurd task of insisting that it couldn't be possible Jesus is simply saying that James and John are going to endure suffering for his sake, or that the murder of James would be a bitter cup for his brother John to drink. No, they have to insist that Mark thinks John was martyred, even though the whole tradition of the Church preserves no such tradition at all. One hears the sound of an ax on the grinding wheel of an agenda, not of a sensible reading of a text.
Yet another criticism of Johannine authorship turns the very sophistication of the gospel against it. Some declare that John bar-Zebedee, a mere fisherman, could not have been an educated Greek-speaking theological genius and therefore could not have written such a theologically sophisticated work.
Here’s the problem: The assumption that a Jewish fisherman living two thousand years ago couldn't be multi-lingual, or educated, or a genius or a contemplative—or all four—is a very fine illustration of what the great Christian writer C.S. Lewis used to refer to as “chronological snobbery”. This is, roughly speaking, the notion that we are, by virtue of our blenders and hi-def TVs, 2000 years smarter than people who lived in Jesus time and that we are therefore comfortably ensconced on the final and permanent platform from which to look down on all human history. It is to forget something a reader of mine puckishly pointed out:
I mean, come on–the Greek text clearly indicates someone who had at least 4 years of Koine Greek in college, and maybe even some in grad school (classics major, perhaps?). And Aramaic, on top of it. That's TWO foreign languages to learn. And it was someone with intimate knowledge of Judaism (religious studies minor?).
How could John have had time to take these courses, much less pay for them? I mean, Hebrew and Bar-Ilan wouldn't even be founded for nearly 2,000 years!
And where'd he pick up all that theology, if it was John? After all, John was spending all his free time running around with Jesus, so he wouldn't have had time to study theology.
Sheesh! To think that a Jewish fisherman in ancient Hellenized Palestine would have had time to learn ancient Greek and Aramaic and theology while he was running around with Jesus…I mean, it's ridiculous!
[In further exchanges, we can argue about the authorship of "Caesar's" Gallic Wars:
How could a mere military commander have time to learn Classical Latin and French geography while he spent all that time encamped on remote Gallic battlefields? He wasn't a professional geographer with a flair for ancient languages, after all!
And the plays of "Shakespeare":
How could a regular guy living just after the Middle Ages, of all times, take the time to learn Shakespearean English? I mean, all those thees and thous—do you expect anyone other than a tenured English professor to manage those?]
In other words, in the zeal to argue John was "just" a fisherman, the critic forgets that Paul was "just" a tentmaker, yet still had plenty of time to get educated. He forgets that native Aramaic-speaking John lived in "Galilee of the Gentiles" and that the normal lingua franca of a tradesman at this crossroads of various civilizations was Koine Greek.
But beyond his language skills, the matter of John's theological prowess is much more acute—and surprising to moderns who think education begins and ends with plump suburbanites. It should be carefully marked that John's gospel makes a rather curious note—and not one anybody would invent: it says that John was "known to the high priest" (John 18:16). That would be Caiaphas, the guy John’s gospel holds accountable for engineering Jesus’ death. John—the supposedly ignorant and uneducated fisherman—was known to the most important theological and political brain in Judea c. 33 AD. And this strongly suggests that John may have spent more time in Jerusalem and gotten more of an education than we think.
The fact is, most of our pop culture picture of John comes from movies full of "humble fishermen" in ragged clothes. But it is quite possible to construct a picture of the fisherman John from the New Testament which leaves room for a man as well-educated as the tentmaker Paul. The fact that the Jerusalem elite thought the apostles uneducated means only that the Jerusalem elite were snobs, which we knew. It's entirely possible that John had studied with rabbis. It's possible he was familiar with the work of his contemporary, Philo of Alexandria, (who has his own notions about the Logos and its relationship with the word of God). It's possible that John, after his apostleship began (or even before), was interested in the philosophy of the pagans. He would have known plenty of them in in Galilee of the Gentiles. Indeed, that may have been exactly what drew him to preach the gospel in cosmopolitan Ephesus. It's possible that he was taught by rabbis in Jerusalem who were interested in the conversation between the Scriptures and the pagan philosophies. All sorts of things are possible. But certainly nothing merits the claim that there is "absolutely no scholarly evidence" that the gospel is substantially the eyewitness testimony of John the apostle.
In sum, if an ancient Jewish tentmaker could be a theologically-well-educated polyglot, so could an ancient Jewish fisherman. All the evidence we possess suggests that this is exactly what John was. At most, it suggests that John's written testimony was assisted by the work of a more polished writer, who himself insists that John is the source of what he's writing. Given that there is not a trace of doubt about this in the early Church, a normal literary historian would take this as very strong evidence that this is John's testimony. Only an agenda-driven conspiracy theorist finds this too difficult to buy.
When all this is said, one last stratagem is sometimes deployed by the critic of Johannine authorship. It goes something like this: Why accept the so-called “internal evidence” of the gospel of John when you don’t accept the Book of Mormon or the Quran?
That argument would have some bearing on the discussion—if we were talking about a sola scriptura claim for the divine inspiration of John's gospel. But in fact we are talking about textual analysis and historic evidence, not concerning the inspiration of a document, but concerning the human authorship of that document. It takes faith to believe that God revealed the New Testament, the Quran or the book of Mormon. But it takes only reason and evidence to believe Mohammed wrote the Quran, Joseph Smith wrote the book of Mormon—or that John is the author his gospel. Such evidences exist both internal to the documents in question, as well as in testimony from external witnesses. It's how we know Julius Caesar wrote his Gallic Wars and it's how we know John wrote his gospel.
What lies behind all this sort of criticism is a scenario like this:
Long ago, sometime between Jesus (whoever he really was) and the rise of the "organized Church", some unknown editors just cooked up a story about Jesus, attributed it to, say, John, and sent it off to random communities of Stupid People who were 2000 years dumber than us. These Stupid People naturally believed without question both that the book was from John and that John was telling the truth, so they started a Church based on this book. They never bothered to check up on any of this, because they were 2000 years more gullible than we Brights. Nor did anybody from the community where John lived ever say, "Hey! John didn't write that!" Nor did John himself ever protest that he'd said nothing of the kind. Fortunately, Brights are 2000 years smarter and these elementary questions occur to them.
In fact, however, the community, not the book, comes first. The book is the testimony, not merely of one man, but of the whole Church. The book was believed because the man was believed. And the man was believed, in part, because he was not one man (like Mohammed or Joseph Smith) claiming a vision and promising earthly pleasures and power, but because he is one of 500 people who bear witness by a life of martyrdom to public events that took place within the living memory of all Israel (1 Corinthians 15:6). That's the meaning of the endorsement at the end of the gospel from the Johannine community ("It is this disciple who testifies to these things and has written them, and we know that his testimony is true" (John 21:24)). It doesn't mean "Dear Gullible Stranger: Read this, believe it, and don't question whether it really came from John. Signed, a Pack of Anonymous Con Men You Can Trust". It means "You guys in the neighboring diocese down the road know John and what he has suffered for the gospel and you know us. We will vouch for the accuracy of this document."
That's why John's gospel propagated so quickly and was so quickly accepted. It's also why other gospels that loudly claimed to be from apostles did not propagate quickly and were not accepted, because ancients weren't stupid enough to accept apostolic authorship just because the document claimed it.
It's also why gospels written by figures of no importance in the rest of the New Testament, such as Mark and Luke, were accepted and attributed to them, even though the documents themselves make no claim to authored by these men. Think about it: If you are going to cook up a gospel, why attribute it to such second stringers?
Answer: the gospels weren't cooked up. They are the works of the people to whom they are attributed. The community remembers who wrote them even when the documents themselves do not say, "By Mark", "By Luke", or "By John". That's the scholarly evidence.