Keep in mind, that liberal historical criticism, even among Catholic scholars (John Meier springs to mind) ironically takes almost a “sola scriptura” approach toward determining authorship. Outside testimony is ignored, and modern presuppositions are often used with only the text itself. This is a bit of a ridiculous standard of historical scholarship that is not used for anything else to my knowledge in regards to other ancient writings. The early fathers are unanimous that John the disciple wrote the gospel. They even say why:
Nevertheless, of all the disciples of the Lord, only Matthew and John have left us written memorials, and they, tradition says, were led to write only under the pressure of necessity. For Matthew, who had at first preached to the Hebrews, when he was about to go to other peoples, committed his Gospel to writing in his native tongue, and thus compensated those whom he was obliged to leave for the loss of his presence. And when Mark and Luke had already published their Gospels, they say that proceeded to write for the following reason. The three Gospels already mentioned having come into the hands of all and into his own too, they say that he accepted them and bore witness to their truthfulness; but that there was lacking in them an account of the deeds done by Christ at the beginning of his ministry. And this indeed is true. For it is evident that the three evangelists recorded only the deeds done by the Saviour for one year after the imprisonment of John the Baptist, and indicated this in the beginning of their account. For Matthew, after the forty days’ fast and the temptation which followed it, indicates the chronology of his work when he says: “Now when he heard that was delivered up he withdrew from Judea into Galilee.” Mark likewise says: “Now after that Jesus came into Galilee.” And Luke, before commencing his account of the deeds of Jesus, similarly marks the time, when he says that Herod, "adding to all the evil deeds which he had done, shut up They say, therefore, that the apostle John, being asked to do it for this reason, gave in his Gospel an account of the period which had been omitted by the earlier evangelists, and of the deeds done by the Saviour during that period; that is, of those which were done before the imprisonment of the Baptist. And this is indicated by him, they say, in the following words: "This beginning of miracles did Jesus "; and again when he refers to the Baptist, in the midst of the deeds of Jesus, as still baptizing in non near Salim; where he states the matter clearly in the words: “For into prison.” Christ which were performed before the Baptist was cast into prison, but the other three evangelists mention the events which happened after that time. One who understands this can no longer think that the Gospels are at variance with one another, inasmuch as the Gospel according to John contains the first acts of Christ, while the others give an account of the latter part of his life. And the genealogy of our Saviour according to the flesh John quite naturally omitted, because it had been already given by Matthew and Luke, and began with the doctrine of his divinity, which had, as it were, been reserved for him, as their superior, by the divine Spirit. These things may suffice, which we have said concerning the Gospel of John.
Eusebius Church History, Book III quoting Clement I
Matthew also issued a written Gospel among the Hebrews(3) in their own dialect, while Peter and Paul were preaching at Rome, and laying the foundations of the Church. After their departure, Mark, the disciple and interpreter of Peter, did also hand down to us in writing what had been preached by Peter. Luke also, the companion of Paul, recorded in a book the Gospel preached by him. Afterwards, John, the disciple of the Lord, who also had leaned upon His breast, did himself publish a Gospel during his residence at Ephesus in Asia. Irenaeus Of Lyons, Against Heresies, Book III Ch. I
We may therefore make bold to say that the Gospels are the first fruits of all the Scriptures, but that of the Gospels that of John is the first fruits. No one can apprehend the meaning of it except he have lain on Jesus’ breast and received from Jesus Mary to be his mother also. Origen. Commentary on the gospel of John Book I, 6.
