Except that it is very unlikely that John wrote the book of John. That is only Christian tradition, it is not based upon facts or evidence. John was the last gospel written. It is the least likely to have any eyewitness testimony in it at all.
Historian Justin Martyr mentioned the gospel of John. Why Everyone Should Believe The Apostle John Wrote The Last Gospel | Reasons for Jesus
1. AN EARLY ROMAN WITNESS
We’ll start with Justin Martyr, who was writing from Rome in around 150 AD.
“In the memoirs [=Gospels], which I say have been composed by the apostles and those who followed them”… Dialogue with Trypho, 103.8.
Some skeptical scholars have suggested that Justin doesn’t know the Gospel of John. This strikes me as a silly notion. For starters, Justin implies that there were multiple Gospels. He says that apostles (plural) wrote them, so that would indicate at least two. Also, in Justin’s writings, he quotes John 3:3. See 1 Apology 61:4: “For Christ also said, ‘Except ye be born again, ye shall not enter into the kingdom of heaven.’
Justin also had a student by the name of Tatian. Just a generation later, Tatian wrote a harmony of the Gospels titled The Diatessaron. In Latin, Diatessaron quite literally means ‘made of four ingredients’. His harmony begins with: “In the beginning was the Word“, quoting John 1:1.