Heidegger's Bible Handbook: Revelation: Authorship

2. Who signified it through an Angel sent to His servant John, the author of the book. That this is John the Evangelist and Apostle is proven. The consensus of the ancients. Why is he called Θεολόγος, the Theologian?

Jesus Christ is said to have signified this Revelation through an Angel sent τῷ δούλῳ αὐτοῦ Ἰωάννῃ, unto His servant John, Revelation 1:1. That this is the same John that wrote the Gospel and Epistles, and so is the Evangelist and Apostle, none of us doubts. For in Revelation 1:2 he is said to bare record of τὸν λόγον τοῦ Θεοῦ, καὶ τὴν μαρτυρίαν Ἰησοῦ Χριστοῦ, ὅσα τε εἶδε, the word of God (which according to the promise he spoke through Christ), and the testimony of Jesus Christ (that Jesus is the Christ, and His precepts, not Antichrist’s, are to be kept), and what he saw. In the written Gospel he did the first; in the Epistles, the second; in the Apocalypse, the third. It is added, that he is called John without qualification; and in Revelation 1:9 he testifies that he was on Patmos, which ancient history attributes to the Apostle John; and the style of familiar forms of speaking (as that he calls Christ the lamb, λόγον, the Word, etc.) agree with the style of the Apostle John (although in a Prophetic books, by inspiration of God narrating visions, he was obliged to speak in a somewhat different style than what he used in the Gospel and Epistles); and the author of the Apocalypse sets himself in opposition to Antichrist, just as John does in his first Epistle; finally, it was appropriate that the most excellent revelation be delivered by the most beloved Apostle. Hence the most ancient men also attributed this book to the Apostle John, for example, Justin in contra Tryphonem; Irenæus in Against Heresies, book IV, sections 37, 50, book V, section 30; Clement of Alexandria in Pædagogo, book II, sections 10, 12; Tertullian, against Marcion, book IV, and elsewhere; Origen in Tractate 12 on Matthew 20; Epiphanius on hæresi 54. And, that this was the opinion of all the Latins, Jerome testifies in his Epistola ad Dardanum. I pass over in silence the Syriac and Arabic inscription and subscript, and also the Greek of the Complutensian Codex, which vindicates this book as consistent with John the Evangelist and Apostle. But, inasmuch as he is called Θεολόγος, the Theologian, in the inscription, it happened that the ancient Christians after Origen called him the Theologian κατ᾽ ἐξοχὴν, par excellence, with much greater reason than the Platonists thus called Orpheus;[1] because he not only spoke of the word of God, or God, like all the other Apostles, but because among the other Apostles most illustriously, and in the very beginning of the Gospel of the Son, ἔλεξε, he declared, and named the Only-begotten Son of God Θεός/God.

[1] In Greek mythology, Orpheus, son of the King of Thrace and the Muse Calliope, was a musician, poet, and prophet, taught by Apollo, the god of prophecy.