You would think that the question of who wrote the Gospel of John would be one of the most settled points about the gospel. After all, the name of the book is “John” just as the other gospels are Matthew, Mark, and Luke. There is also historical evidence and testimony for Johannine authorship. The debate over who wrote this gospel was seemingly resolved by the close of the second century of the Christian Era (CE). This was the status quo for sixteen centuries, only to be revisited during the Enlightenment and at various times since then.
If we assume that one of the twelve disciples of Jesus wrote the gospel, deducing the identity of the author is fairly straightforward. We can eliminate one of the twelve immediately: Judas Iscariot. We can also remove two others, Matthew and Mark from the list since they also wrote gospels. A reading of the Gospel reveals that the writer was more intimately aware of the events, conversations, and thoughts of Jesus than most of the other disciples. This eliminates the peripheral members of the group: Andrew, James the Less, Simon the Zealot, Judas (not Iscariot), Philip, Thomas, and Bartholomew (thought to be Nathanael in the Synoptics). With the elimination of these nine, the remaining candidates are Peter, James, and John. Acts 12:1-3 eliminates James, the brother of John, as the author when it explains that he became a martyr before Peter. The remaining possible authors are now Peter and John. As Peter wrote two epistles, 1 and 2 Peter, it seems likely people would recognize him as the author of the gospel if he had written it. As a note, many believe Peter is a significant contributor to the writing of the gospel of Mark. John 21:19-24 indicates by the tense of some of the words that Peter had become a martyr at some previous point in time. By process of elimination, the disciple to write the fourth Gospel is the Apostle John.
B. F. Wescott uses the same approach. While some of his supporting material is open to criticism, the overall argument for Johannine authorship is strong. He determines the author to be Jewish because of his considerable knowledge of the Old Testament as indicated by statements in John 10:34-36, 12:40, and 17:12. Other indicators that the author of the Gospel is Jewish is his knowledge of the festivals, the Law, the relationship between the Jews and Samaritans, as well as seemingly trivial details such as the tassels or fringes (tzitziyot) that Jesus wore on the four corners of His robe. John “accurately understands Jewish customs, is steeped in the Old Testament, is aware of finer points of distinctions among pre-70 Jewish sects, and is concerned to demonstrate Jesus as the true fulfillment of the Law and the numerous rituals and institutions of Judaism.”  Also, the author is aware of the religious and political climate in Judea, Samaria, and Galilee. The author is knowledgeable of social norms and protocols at the time of Jesus and social paradigms such as honor and shame, patron and client relationships, and the perception of limited resources. He is proficient in scriptural hermeneutics. John 4:9, 7:35, and 11:59-50 reveals that he is aware of the low view held of women and the deep animosity between Jews and Samaritans. The author knows of the Passover, the most significant event in the Jewish calendar at the time of Jesus. This is seen in John 2:23, 13:1, and 18:28. The author is knowledgeable of the Old Testament connection between the Messiah and Elijah. Also, he knows Scripture sufficiently well to compare Jesus to Abraham and Moses, two of the most revered Jewish patriarchs.
Another illustration of the author’s knowledge of things Jewish is his explanation of the symbolism of the manna God gave the Jews during the Exodus and the bread from heaven. Several “studies have demonstrated how John’s references to geography and topography prove remarkably accurate, particularly in and around Jerusalem.”  Verses that illustrate this knowledge of the city of Jerusalem include John 5:2-3, 9:6-7, and 19:17. Archeologists have found the pool with the five porticoes mentioned in 5:2-3, which is yet another validation of Scripture. According to this gospel, Jesus and His disciples went to Jerusalem at least five times during His ministry, and these trips, taken in a relatively short period, would have provided sufficient opportunity to acquire the knowledge necessary for John to accurately recount places and events in Jerusalem as found in John 4:5-6, 11:5-7, and 20:19. The author also records events as an eyewitness. Similar verses include 3:22-23, 6:22, 7:14, and 12:1. The author is also aware of conversations among members of the small band of men as shown in John 4:33, 16:17-18, and 20:25. It is John who leans against Jesus at the Last Supper to ask who would betray Him. John is also one of the two disciples, the other being Peter, at the house of the high priest during the arrest and trial of Jesus. John is the one whom Jesus asks to care for His mother, Mary, and is likely the eyewitness who sees the water and blood come from the side of Jesus after the Roman soldier uses his spear to pierce the body of Jesus. It is John who recognizes Jesus calling from the shore to the disciples in the boat after the resurrection.
Other verses in the gospel reveal that John “is present at key points in the story of Jesus”  and lends credibility to what he writes. Cynical critics also see what John claims to be eyewitness testimony actually to be John’s literary attempt to invent intimacy or personal knowledge. “But it is difficult to think that that is an adequate explanation of all the passages adduced. Sometimes these concerns the time of day at which a thing happened, or perhaps there is a link with one of the feasts. Place names are brought in very naturally, and often for no apparent reason other than that it was there that the incident happened.”  While this does not prove that John wrote the fourth gospel, it does reveal that he had first-hand knowledge from which to recollect. Matthew 17:1, Mark 9:2 and Luke 9:18 note that he, James, and Peter saw the transfiguration. The same three witnessed the raising of the daughter of Jairus and were also with Jesus in the Garden of Gethsemane as seen in Mark 5:37, Mark 14:33, and Luke 8:51.
The author is sometimes known as the beloved disciple, and this special relationship is apparent throughout the gospel. It is probable that this individual was Peter, James, or John. It was likely not Peter since the gospel mentions him by name. This leaves James and John. However, it is most likely not James since he was martyred fairly early (Acts 12:1-2). Thus, the logical assumption is John. Some question the designation of “beloved disciple,” arguing that nobody would refer to himself in such a way; the title had to come from someone else. This argument is essentially specious, for this designation in no way means Jesus loved the other disciples any less but simply personalizes the writer. The Apostle Paul uses the same technique when referring to himself in the first person and as the chief of sinners. Also, there are several references to the “other” disciple or “another” disciple, as seen in John 18:15-16, 20:2-4, and 20”8, identified as the beloved disciple. These verses, as well as supporting passages from the Synoptics concerning the Apostle John and his knowledge of the ministry of Jesus, do not create an ironclad statement for Johannine authorship. They do, however, make a compelling argument.
You can also fairly confidently deduce that the author of this gospel knew Greek. Some ask whether he was sufficiently proficient in the written aspect of the language to craft the gospel. Some suggest the Gospel was first written in Aramaic and later translated into Greek, but evidence to support this idea has yet to be discovered, and the great majority of scholars reject this premise. While Aramaic and Semitic aspects are discernible in the gospel, this is expected, given that Jesus and the disciples spoke Aramaic, and the main character was Jewish. Another argument supporting Johannine authorship is the fact that the gospel never names the author. It is dubious to believe anyone else who could have written the gospel would not refer to John by name. This is significant because the gospel typically identifies individuals; a case in point is the author’s explanation that the Judas, who asks Jesus a question is not Judas Iscariot, the Judas, who betrays Him.
A potential problem to authoritatively claim the Apostle John as the author lies in the fact that the earliest manuscripts do not attribute the writing to the Apostle John. “Although the hypothesis of the apostolic authorship of the gospel is regularly rejected in recent Johannine scholarship, the hypothesis has never been decisively refuted and continues to be at least as plausible as alternative explanations.”  The large amount of Scriptural evidence, both within the fourth gospel and in Matthew, Mark, and Luke, however, does offer a strong argument that the Apostle John is, in fact, the author of the fourth gospel.
The last chapter of the gospel is another point of major contention concerning authorship. Chapter twenty seems to close nicely and serves as a fitting ending to the gospel. Now, after this stirring conclusion here comes chapter twenty-one. This chapter tells of the last time Jesus was with the disciples and contains the narrative of Jesus asking Peter three times whether he loves Him. John 21:24-25 also denies speculation that the Apostle John was to live until the return of Jesus. The last two verses are the strongest indicators that the Apostle John did not write this chapter. These verses are a testimony to the disciple and once again close the gospel in a powerful manner. It is interesting to note that the “two stages of the conclusion are parallel, but not repetitive. At every point where they are parallel, the second stage of the conclusion takes the matter on a stage further from the first.”  As an example, the first ending of the gospel (John 20:30-31) refers to many other signs Jesus did that are not in the gospel; in the second ending, the writer expands this thought (John 21:25) to note that all the books of the world could not record everything about Jesus. While there is no consensus as to who wrote the last chapter, be it a single individual or a group, three suggestions are worthy of mention. The first is that the Apostle John wrote the first twenty chapters, and after his death members of the Johannine community of churches wrote the last chapter with the intent of resolving a couple of troublesome issues. In doing so, they thought to affirm the veracity of the gospel and wrote another closing statement. Another suggestion is there were two individuals who collaborated on the writing of the fourth gospel. An editor wrote the gospel and identified the author as the Beloved Disciple, who was the source of his material. It was the second person, the editor, who, in conjunction with the Johannine church, wrote the last chapter. Another suggestion is three individuals provided different areas of expertise to the writing of the gospel. The first person of the three was the Apostle John, and he provided the source material, likely orally sharing his experiences, to the evangelist, who wrote the Gospel. The third person was an editor who corrected any literary or grammatical problems in the writing. For the writing of the last chapter, the evangelist or the editor wrote the material, likely under the guidance of elders from the Johannine churches.
Borchert offers an interesting suggestion. He advocates that the Apostle John wrote the first version of the gospel and the three epistles to the Johannine community. After John wrote the first draft, he and others edited and reordered some of the chapters. The last section the Apostle John wrote was chapter twenty. Someone else in the Johannine community wrote the last chapter and included testimonies about the author of the gospel. There is a clear example of this other writer in 21:24-25. Thus, as Craig Keener writes, “John is the author of the Gospel as we have it, to whatever degree he might have permitted his scribe or scribes freedom in drafting his sermonic material.”  This is the only instance, referring to chapter twenty-one, in which I can support the concept that the Apostle John did not write a portion of the gospel. Evidence to conclusively prove that anyone else but John wrote the gospel does not exist. This position concerning the last chapter of the gospel is strong and defensible but also entails some degree of risk. “Once this rampart (that another person contributed to the contents of the gospel) has been breached the enemy is within the gate; there can be no further defense of integral unity as a matter of principle, and the familiar argument that there is no manuscript evidence, either here or elsewhere, of editorial insertions or modifications loses all its force.”  Rudolf Bultmann illustrates in his analysis of John's gospel the exact situation that breaches this rampart when he writes “if chapter twenty-one betrays the redactional influence, we must ask the further question whether other parts of the gospel are due to the redactor. The question also must be answered in the affirmative. An example is the passages that related to the Lord’s Supper and baptism. Sentences that express an apocalyptic expectation of the future are also due to the redactor. And lastly, individual explanatory glosses that clearly break into the context should be viewed as editorial.” 
One should realize that Ephesus while it sounds logical and appealing, is not the only feasible place in which the Gospel of John could have been written. Another viable location is Antioch in Syria. The early church thrived in the region, and several of the Apostle Paul’s missionary journeys occurred in the vicinity. Additional facts in support of Antioch include Theophilus, Ignatius of Antioch is the first in a prominent position to declare the Apostle John the author of the gospel; thus the suggestion that it was written in Syria has some merit. Just as there is no evidence that John did not move to Ephesus at some point after Pentecost, there is also no evidence that he did not move to Antioch. Both are major centers and are within several hundred miles of each other, so it is entirely possible the Apostle could have resided in both locations. Others suggest Antioch as the place of writing because of a perceived lack of influence from the Judaism found in Jerusalem and Judea, and the tendencies in Antioch toward Palestinian Judaism, which is slightly different. We should note, of course, that one does not have to be at a certain location to know details about the geography and sites of a region or city.