A man who says his name was John. That's the whole of what can be established, and it's less than you were told.
He names himself four times. At 1:1, at 1:4 — “John to the seven churches that are in Asia” — at 22:8, and most fully at 1:9:
I, John, your brother who share with you the persecution and the kingdom and the endurance in Jesus, was on the island called Patmos because of the word of God and the testimony of Jesus.
Now notice everything he doesn't say. He never calls himself an apostle. He never claims to have known Jesus, to have been present at anything in the Gospels, or to be the son of Zebedee. He calls himself a servant and a brother.
Whoever wrote this made no claim to apostolic authority at all. That was supplied later, by readers who needed the book to have it.
The oldest objection is the best one. In the third century, Dionysius of Alexandria argued that the author of Revelation could not be the author of the Fourth Gospel, and his reasoning survives in Eusebius. He noticed first that the writer of Revelation names himself immediately, while the evangelist “nowhere else subjoins his name.” Then he went to the Greek. The Gospel and the Epistle, he said, are written “with the greatest elegance” and are “very far indeed from betraying any barbarism or solecism.” The author of Revelation, by contrast — his “dialect and language are not of the exact Greek type, and that he employs barbarous idioms, and in some places also solecisms.”
That's a bishop in the 200s doing careful stylistic criticism and reaching the conclusion modern scholarship reached again with better tools. Revelation's Greek is irregular in ways the Fourth Gospel's is not, full of constructions that look like a writer thinking in a Semitic language and writing in Greek.
The majority critical position today is that the author of Revelation is not the apostle and not the author of the Gospel of John. The candidates are a figure the second-century writer Papias called John the Elder, or an otherwise unknown Jewish-Christian prophet working among the churches of Roman Asia.
Worth untangling, because it comes up: John the Baptist has nothing to do with this book. He was executed before the crucifixion, and nobody in the field has ever proposed him. The conflation that actually happens is between John the apostle, John the Evangelist, John the Elder, and the John of Patmos — four figures that church tradition collapsed into one man over several centuries.
So the book at the end of your Bible was written by someone about whom the only verifiable fact is a first name. Anyone who tells you more than that is filling in blanks.
I write about what’s actually in the Bible, as opposed to what people are told is in it. The free guide below covers twelve verses that are in your Bible and were never in the sermon.
There’s a novel too — The War Against the End of Days, an apocalyptic reimagining of Revelation.