Heidegger's Bible Handbook: 2 John: Authorship
1. The Author of the Epistle is the Apostle John, not an Ephesian Elder differing from him, as Grotius imagines, whose arguments are refuted.
That this Epistle, and that which follows, were written, not by the Apostle John, but by some Ephesian Elder by the name of John, Hugo Grotius undertakes to prove from the testimonies of Eusebius and Jerome. But Irenæus, more ancient than both, assigned that to the Apostle John without hesitation, book I against Heresies, chapter 13: John, the disciple of the Lord, has enlarges the condemnation against them, not willing that a greeting be spoken to them by us: For whoever says to them, Greetings, etc., verse 11. And no reason appears, why that should be claimed for any other than Saint John the Apostle, with the style, words, and thoughts being fully consistent with him. For, that he calls himself πρεσβύτερον, the elder, does not argue that the Epistle was not written by the Apostle John, as Grotius insists. He callse himself the elder, if not on account of the dignity belonging to all the Apostles, certainly on account of his age. For as an old man, and with his age pressed beyond endurance and his Apostolic duty fulfilled, and like a parent to children, he wrote the Epistle. It was also customary for Saint John to avoid mentioning, rather than to express, his name, whether for fear of persecution, or for other reasons. Neither does it hinder that these Epistles were not immediately received into the canon, or expressed in vernacular versions. For they were not marked with a public character, nor with the name of the author. Nevertheless, it is evident that they were a short time thereafter adopted into the canon, and that this inscription was prefixed by the Church: Ἰωάννου τοῦ Ἀποστόλου ἐπιστολὴ καθολικὴ δευτέρα, the Second Catholic Epistle of the Apostle John. Moreover, the Most Illustrious Gomarus, pages 741, etc., solidly vindicated the authority of this and the following Epistle against some ancients, and also more recent men, Cajetan, Luther, Flaccius, Hafenreffer,[1] etc.
[1] Matthias Hefenreffer (1561-1619) was a German Lutheran theologian, serving as Professor of Theology at Tubingen (1592-1619).