My research gives evidence that John was the writer:
(1) The writer of the book was evidently a Jew, as is indicated by his familiarity with Jewish opinions.Joh 1:21; 6:14; 7:40; 12:34.
(2) He was a native dweller in the land of Palestine, as is indicated by his thorough acquaintance with the country. The details mentioned concerning places named indicate personal knowledge of them. He referred to Bethany across the Jordan (Joh 1:28) and Bethany near Jerusalem. He wrote that there was a garden at the place where Christ was impaled and a new memorial tomb in it, that Jesus spoke in the treasury as he was teaching in the temple, and that it was wintertime, and Jesus was walking in the temple in the colonnade of Solomon.
(3) The writers own testimony and the factual evidence show that he was an eyewitness. He names individuals who said or did certain things (Joh 1:40; 6:5, 7; 12:21; 14:5, 8, 22; 18:10); he is detailed about the times of events (4:6, 52; 6:16; 13:30; 18:28; 19:14; 20:1; 21:4); he factually designates numbers in his descriptions, doing so unostentatiously.1:35; 2:6; 4:18; 5:5; 6:9, 19; 19:23; 21:8, 11.
(4) The writer was an apostle. No one but an apostle could have been eyewitness to so many events associated with Jesus ministry; also his intimate knowledge of Jesus mind, feelings, and reasons for certain actions reveals that he was one of the party of 12 who accompanied Jesus throughout his ministry. For example, he tells us that Jesus asked Philip a question to test him, for he himself knew what he was about to do. Jesus knew in himself that his disciples were murmuring. He knew all the things coming upon him. He groaned in the spirit and became troubled. The writer was also familiar with the apostles thoughts and impressions, some of which were wrong and were corrected later.
(5) Additionally, the writer is spoken of as the disciple whom Jesus used to love. He was evidently one of the three most intimate apostles that Jesus kept nearest to him on several occasions, such as the transfiguration and the time of his anguish in the garden of Gethsemane. Of these three apostles, James is eliminated as the writer because of his being put to death about 44 C.E. by Herod Agrippa I. There is no evidence whatsoever for such an early date for the writing of this Gospel. Peter is ruled out by having his name mentioned alongside the disciple whom Jesus used to love.
Authenticity:
The Gospel of John was accepted as canonical by the early Christian congregation. It appears in nearly all the ancient catalogs, being there accepted without question as authentic. The epistles of Ignatius of Antioch (c. 110 C.E.) contain clear traces of his use of Johns Gospel, as do also the writings of Justin Martyr a generation later. It is found in all the most important codices of the Christian Greek Scriptures the Sinaitic, Vatican, Alexandrine, Ephraemi, Bezae, Washington I, and Koridethi codicesas well as in all the early versions. A fragment of this Gospel containing part of John chapter 18 is contained in the John Rylands Papyrus 457 (P52), of the first half of the second century. Also parts of chapters 10 and 11 are found in the Chester Beatty Papyrus No. 1 (P45), and a large part of the whole book is found in the Bodmer Papyrus No. 2 (P66) of the early third century.