Is John the Author of His Gospel? same essay, alternative link Mark Shea argues brilliantly. I will add that the Gospel is written by someone who has come to use "Jew" as synonym for the pharisaic sect rejecting Christ, and yet remembers that Christ himself way back when he walked on earth used the word purely as an ethnonym referring to his own, to the people he stemmed from and whose rightful king he was and remains.
A quote that will be a favourite: "In sum, if an ancient Jewish tentmaker could be a theologically-well-educated polyglot, so could an ancient Jewish fisherman."
Jews tend to be polyglots and theologically well-educated, whatever their religion or the age they live in ... even if very Gentile in ancestry.
You may of course add - if you're getting serious - that all the twelve, except Judas Ischariot, that is the eleven, were instructed in the scriptures by Christ when he rose from the dead and appeared to them. And four at least of them - Peter, Andrew, James and John - had been disciples to St John the Baptist, who was a prophet and also a son of a high priest. And John had also been foster son of the blessed Virgin and Theotokos for all the years between her Sons Crucifixion and her blessed Dormition. And he had received further instruction on Patmos - as set down in his Apocalypse - before writing his Gospel, possibly before the Epistles as well.
Others, grasping at straws, may claim that Mark 10:38-39…
But Jesus said to them, "You do not know what you are asking. Are you able to drink the cup that I drink, or to be baptized with the baptism with which I am baptized?" And they said to him, "We are able." And Jesus said to them, "The cup that I drink you will drink; and with the baptism with which I am baptized, you will be baptized."
…implies that both James and John suffered a martyr's death, contradicting John 21:22-23.
It actually "only" implies both suffered martyrdom. St John was martyred by boiling oil, but survived it miraculously before going to Patmos. He has a martyr's merit, but not a martyr's death. As is also true for his second mother, the Blessed Virgin Mary, whose martyrdom was watching the crucifixion: both survived by miracle something which would otherwise have killed them (at least that is what I read in The Glories of Mary, by Alfons Liguori).