Verse 9:[1] I John, who also am your brother, and (Phil. 1:7; 4:14; 2 Tim. 1:8) companion in tribulation, and (Rom. 8:17; 2 Tim. 2:12) in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, (Rev. 6:9; 1:2) for the word of God, and for the testimony of Jesus Christ.
[I, John] Namely, that one already designated (Grotius). He expresses his name, for prophecy requires the authority of the writer (Pareus).
[Your brother] That is, of the same faith (Grotius, similarly Ribera, Menochius, Tirinus).
John, who also am your brother; the same mentioned Revelation 1:4, the apostle of Jesus Christ, yet he disdaineth not to call those his brethren whom his Lord so called.
[1] Greek: Ἐγὼ Ἰωάννης, ὁ καὶ ἀδελφὸς ὑμῶν καὶ συγκοινωνὸς ἐν τῇ θλίψει καὶ ἐν τῇ βασιλείᾳ καὶ ὑπομονῇ Ἰησοῦ Χριστοῦ, ἐγενόμην ἐν τῇ νήσῳ τῇ καλουμένῃ Πάτμῳ, διὰ τὸν λόγον τοῦ Θεοῦ καὶ διὰ τὴν μαρτυρίαν Ἰησοῦ Χριστοῦ.
Dr. Dilday's Lecture: "I, John"
https://www.sermonaudio.com/sermoninfo.asp?SID=102509216457
1) Introductory review
a) General inscription
b) Particular inscription
2) Analysis (v. 9a)
a) Introduction
b) “I John”
i) Importance of John’s identity
ii) Apostolic authorship
(1) External testimony
(a) Justin Martyr (mid-second century)
Dialogue with Trypho 81: “…[T]here was a certain man with us, whose name was John, one of the apostles of Christ, who prophesied, by a revelation that was made to him, that those who believed in our Christ would dwell a thousand years in Jerusalem; and that thereafter the general, and, in short, the eternal resurrection and judgment of all men would likewise take place.”
(b) Irenaeus (turn of the third century)
Against Heresies 4:20:11: “John also, the Lord’s disciple,…