A. Authorship: Who wrote it?
First John joins Hebrews in being the only two books in the NT with no introductory announcement as to their author. It is an extremely personal letter but nowhere do we find explicit reference to its author (the author of the 2nd and 3rd epistles calls himself "the Elder"). There are, however, several clues which indicate that the author was John the Apostle who also penned the fourth Gospel, the Revelation, and the two subsequent epistles.
1. External Evidence
The early tradition of the church testifies to both the canonical status of the letter and its Johannine authorship. The earliest reference to the epistle is found in Polycarp (d. 155 a.d.), although he does not attribute the book to John (nor, for that matter, to anyone else!). The first to speak of a "Johannine epistle" was Papias of Hierapolis in the middle of the 2nd century. In Irenaeus (130-200 a.d.) the First and Second Epistles are clearly attributed to John, apostle of Christ and author of the fourth gospel. Further confirmation of Johannine authorship is found in the writings of Clement of Alexandria, Tertullian, Cyprian, Origen, and Eusebius (see Kruse for the text of these and other citations). Guthrie concludes:
"This evidence is sufficient to show that from very early times the Epistle was not only treated as Scripture but was assumed to be Johannine, in spite of the fact that no specific claim to this effect is made by the writer himself" (NT Introduction, 865).