First, consider the author’s use of specific, vivid detail. I cannot begin to list all of these, but just look at the details at the beginning of the arrest scene in the Garden of Gethsemane:
Judas then, having received the...cohort and officers from the chief priests and the Pharisees, came there with lanterns and torches and weapons. So Jesus, knowing all the things that were coming upon Him, went forth and said to them, “Whom do you seek?” They answered Him, “Jesus the Nazarene.” He said to them, “I am He.” And Judas also, who was betraying Him, was standing with them. So when He said to them, “I am He,” they drew back and fell to the ground. Therefore He again asked them, “Whom do you seek?” And they said, “Jesus the Nazarene.” Jesus answered, “I told you that I am He; so if you seek Me, let these go their way,” to fulfill the word which He spoke, “Of those whom You have given Me I lost not one.” Simon Peter then, having a sword, drew it and struck the high priest’s slave, and cut off his right ear; and the slave’s name was Malchus. (John 18:3-10)
The arresting officers had both lanterns and torches. Peter cut off the servant’s right ear (a detail also found in Luke). The servant’s name was Malchus. Consider, too, the vividness of the reference to Judas: Judas “was standing there with them.” The “them” in question is, of course, those coming to arrest Jesus. While John, supplementing the Synoptics, does not report the kiss by which Judas betrays Jesus, he does report the fact that might otherwise seem obvious--that Judas was standing with the “bad guys.” The author has a picture stamped on his mind of Judas standing on the wrong side at that fateful moment and, presumably, falling backward with them as well.
Or at the cross, where the Synoptics (e.g., Mark 15:36) state that a bystander put a sponge of vinegar (or sour wine) on a reed to offer to Jesus (already vivid enough), John 19:29 is even more specific, noting that it was a hyssop. At the same place, John specifies that there was a jar full of sour wine standing nearby. The Beloved Disciple, we are told in John 19:26-27, was standing by the foot of the cross. If he is the one who gave us these memories, he was standing right by the jar.
Morris addresses the claim that such details indicate only the artistry of the author:
Sometimes when attention is drawn to the life-likeness of a scene in this Gospel the retort is that this shows no more than the skill of the author as a dramatist. But what could be his motive for setting down this kind of conversation? To reply that this gives an air of verisimilitude scarcely meets the case, for authors in the first century were not given to that kind of verisimilitude. It was foreign to their methods, and we should not read back our ideas into their day.
Morris is quite right here. The literally realistic work of fiction, filled with vivid, lifelike detail, is a creation of a much later time period than that of the Fourth Gospel.
A surprising connection
Next, go back to that passage about Jesus’ arrest. Jesus asks that the others be released if the arresting soldiers are seeking him. And here the narrator makes a surprising connection. He says that Jesus said this so that his own word might be fulfilled, “Of those whom You [the Father] have given Me I lost not one.” This is a reference to John 17:12, “While I was with them, I was keeping them in Your name which You have given Me; and I guarded them and not one of them perished but the son of perdition...”
Surely if we read that part of Chapter 17, famously known as Jesus’ “high priestly prayer” for his disciples and for all believers, we would initially interpret that verse as referring to guarding the disciples spiritually rather than physically. When you stop to think about it, it is surprising that the narrator should think of this as fulfilled by Jesus’ attempt to protect them from physical arrest on the night of his own betrayal.
But think how much more surprising it would be if Jesus never said these words at all--the prayer in John 17 and the request to let the others go in John 18. If there is one area more than any other where scholars cast doubt upon John’s historicity, it is in reporting the words of Christ. Again and again we are told that Jesus in John sounds too much like the narrator himself, that the narrator has significantly embellished, elaborated, and crafted Jesus’ words and discourses, and even that the evangelist thought this was right to do because he was guided by the Holy Spirit! John 17 is the perfect place for this sort of alleged “crafting,” since it is a relatively longer, uninterrupted chunk of Jesus’ speech, full of theology and profound musings. But suppose that John did substantially craft the prayer in Chapter 17, including that statement that Jesus has guarded his disciples so that only one of them perished. Would we not expect that the author would make a more theological, spiritual use of the verse? If he were going to refer to it later, and if he had (in essence) made it up and put it into Jesus’ mouth, I would expect that he would bring it back up, perhaps, in reference to the restoration of Peter in John 21. Or maybe when Jesus overcomes Thomas’s doubts of the resurrection in John 20--that it might be fulfilled which Jesus spoke, “I was keeping them in your name which you have given me.” But no: John applies it here, to Jesus’ attempt to secure the physical freedom of the disciples on that fateful Thursday night.
Or look at the matter from the perspective of the account of the arrest. What if John had invented that lofty request to arrest Jesus and let the disciples go? Would we expect John to connect it back to 17:12? It would make better literary “copy” to connect it to John 10:11: “I am the good shepherd; the good shepherd lays down His life for the sheep.”
This “worldly” interpretation of John 17:12 by the narrator in John 18:8-9 points to the historicity of both utterances by Jesus. It is a slight indication, and a subtle one, but it does not accord well with the picture of a “John” who invents things for Jesus to say for theological and literary reasons. It fits much better with an evangelist who records what he believes to be true, because he believes that it happened, and who then tries to make connections between the sayings of Jesus, putting these connections openly in his own voice.
Third, consider the fact that John’s Gospel contains many unexplained allusions. In an unexplained allusion, the narrator or a speaker says something that the audience is unlikely to understand, but it is never explained. The interesting thing about unexplained allusions is that they make for poor fiction. Even if we were anachronistically comparing a Gospel to modern, realistic fiction, unexplained allusions wouldn’t fit. They would not serve any literary purpose.
For example, in John 7:37-38, we find this:
Now on the last day, the great day of the feast, Jesus stood and cried out, saying, “If anyone is thirsty, let him come to Me and drink. He who believes in Me, as the Scripture said, “From his innermost being will flow rivers of living water.’”
Commentators have conjectured and puzzled about which Old Testament Scripture might be meant here. Here are some of the commentaries. The bottom line is that no one really knows if Jesus was referring to some one particular Scripture and, if so, which one. And that is just the point. If John were prone to construct Jesus’ words, as so many scholars imply, why would he “make” Jesus allude to a Scripture that no one can find? That serves no good literary purpose at all. Morris puts the point well:
One [problem] is the notorious difficulty of knowing what passage of the Old Testament Jesus had in mind. But the very fact that the difficulty can arise is, of course, evidence for the genuineness of the passage. As [John H.] Bernard points out, ‘...The fact that we cannot precisely fix the quotation makes for the genuineness of the reminiscence here recorded. A writer whose aim was merely to edify, and who did not endeavour to reproduce historical incidents, would not have placed in the mouth of Jesus a scriptural quotation which no one has ever been able to identify exactly.’ This must be taken with full seriousness. It is intelligible that Jesus cited Scripture in an unusual fashion. It is not intelligible that someone who was manufacturing the incident would affirm that Jesus ascribed certain words to Scripture, but do it so badly that no one has been able to find the passage.
Here is an unexplained allusion that I have noted from John 3. John the Baptist’s disciples are about to come and raise a concern about the fact that Jesus is (through his disciples) baptizing more people than John the Baptist is. This leads into the famous place where John the Baptist says, “He must increase, but I must decrease” (John 3:30). But notice the lead-in:
John also was baptizing in Aenon near Salim, because there was much water there; and people were coming and were being baptized—for John had not yet been thrown into prison. Therefore there arose a discussion on the part of John’s disciples with a Jew about purification. And they came to John and said to him, “Rabbi, He who was with you beyond the Jordan, to whom you have testified, behold, He is baptizing and all are coming to Him.” (John 3:23-26)
Aside from the realistic note mentioning Aenon near Salim (John’s Gospel is rightly famous for the historical specificity of its topography) and the meticulous concern for detail with which the narrator mentions that John had not yet been thrown into prison, these introductory verses have another mark of truthfulness: We never learn what the dispute was about purification! Not one syllable of a clue does John give us about this. Was this person a follower of Jesus? Was he saying that people should be baptized by Jesus’ disciples rather than by John the Baptist? Did Jesus’ disciples use some other method or gestures when they baptized from that of John the Baptist? We have no idea. We can only guess, and we have every reason to think that the Gospel’s first readers--probably Christians, including many Gentiles, in relatively far-away Asia Minor--would have had no better theory than we do. But therein lies the realism of the passage. This unexplained allusion, once again, would make poor fiction. The obscure “dispute with a Jew about purification” foreshadows nothing, symbolizes nothing, and furthers no theological or literary agenda. It is the perfect fifth wheel. And for that reason it is all the more likely to be real. Indeed, if (as may be the case) John the evangelist was originally a disciple of John the Baptist before he followed Jesus, he may well have witnessed this scene himself. When retelling it years later, he would have remembered the dispute that occasioned the appeal to John the Baptist only because it gave rise to such memorable and important comments by the Baptist and not for any other reason. And it never occurred to him to pause in his Gospel and explain it.
What all of this resembles more than anything else is oral history--memoirs told by someone who was present. These sorts of unexplained details, mentioned just because they are true, are characteristic of honest narrative but not of crafted and partially fictionalized literary effort.
This is the voice of John, the man who saw.
 Leon Morris, Studies in the Fourth Gospel (Grand Rapids, MI: Eerdmans Publishing Company, 1969), p. 199.
 Ibid., p. 156.
 Ibid., pp. 159-160.