The book we call 'John' was not written by the Disciple John.
How do we know? Many reasons. Let's explore 3 for now.
Contradictions between John and the Synoptic Gospels (Matthew Mark Luke) cannot be reconciled.
1. In John, John the Baptist says he is not Elijah.
John 1.21 "And they asked him, 'What then? Art thou Elijah?' And he saith, 'I am not.'"
In the Synoptics Jesus says he is Elijah.
The Angel, Quoting the Prophecy in Malachi said to John's father concerning John:
"And he shall go before him in the spirit and power of Elijah, to turn the hearts of the fathers to the children, and the disobedient to the wisdom of the just; to make ready a people prepared for the Lord".
Jesus further makes it clear that the wicked killed Elijah when they killed John:
Mark 9:11 - 9:13
"And they asked him[Jesus], saying, 'Why say the scribes that Elijah must come first?'
And he answered and told them, 'Elijah verily comes first, and restores all things; and how it is written of the Son of man, that he must suffer many things, and be set at nothing.
But I say unto you, That Elias is indeed come, and they have done unto him whatsoever they wanted, as it is written of him.'"
“Assuredly, I say to you, among those born of women there has not risen one greater than John the Baptist; but he who is least in the kingdom of heaven is greater than he. And from the days of John the Baptist until now the kingdom of heaven suffers violence, and the violent take it by force. For all the prophets and the law prophesied until John. And if you are willing to receive it, he is Elijah who is to come. He who has ears to hear, let him hear!
"Jesus answered and said to them, 'Indeed, Elijah is coming first and will restore all things. But I say to you that Elijah has come already, and they did not know him but did to him whatever they wished. Likewise the Son of Man is also about to suffer at their hands.' Then the disciples understood that He spoke to them of John the Baptist."
2. Jesus' ministry in John was 3 years. We know this because there are 3 Passovers.
In the Synoptics his ministry is one year. This was prophesied long ago:
Luke 4.18 "The Spirit of the Lord [is] upon me, because he hath anointed me to...preach the acceptable year of the Lord." - Jesus
3. The Jesus in the book called 'John' is completely different. There is almost NO resemblance between the real Jesus in Matthew Mark and Luke, and in 'John'.
Jesus said: "Why do you call me good? There is ONE who is good and that is God". Mark 10.18
Compare this humility to the Jesus in John and there is no comparison.