Excellent post by Dr. Timothy McGrew on the authorship of John's gospel:
The implication of John actually writing the fourth gospel is huge. If John actually wrote John, it nullifies hallucinations, the metaphor hypothesis, and the non-bodily resurrection theory because of the contents of the book itself. It would be impossible for him to hallucinate every resurrection appearance and every miracle he reported. The metaphor theory doesn't work because specific numbers of items and specific feast days mentioned, and specific conversations are elucidated on those days. Furthermore, non-bodily resurrection is refuted because of the empty tomb and the appearances.
The scholars concede that the disciples didn't lie about the resurrection. In this post right here, I show at length that it is extremely unlikely that the disciples lied about the resurrection. John, being an inner circle disciple, is also unlikely to lie for the same reasons. So the implication of John writing the fourth gospel is that Jesus performed real miracles and rose from the dead.
Furthermore, John's gospel portrays a Jesus who claims equality with God and seems to threaten condemnation to those who refuse to believe. He also promises eternal life to everyone who believes. This leaves no room for a Jesus who is a non-threatening and a merely human prophet. John's gospel forces a person to make a very decisive and emotionally deep decision about who Jesus is.
All of this very easily explains at least one reason why almost all non-Christian scholars want to reject traditional authorship of the fourth gospel...........
Conceding the authorship of John comes rather close to conceding the Christian religion itself......