Introduction
1. Authorship. The question is bound up with that of the authorship of the other Johannine books, both as regards internal and external evidence: see especially Introductions to the Gospel and to the Second and Third Epistles.
(a) Internal Evidence. The witness of the book itself to its authorship is sufficiently strong. The writer speaks with authority, as an Apostle would. He claims to have firsthand knowledge of the facts which underlie the gospel message (1 John 1:1). The tone and teaching of the letter suit the circumstances to which Christian tradition assigns it; they are such as we should expect from the aged St. John, writing to his disciples a last message regarding the truths enshrined in his Gospel.
When the Epistle is compared with the Gospel of St. John, the conclusion that the two books are the work of one hand is well-nigh irresistible. The style, the language, the thought of the Epistle reflect the features of the corresponding elements of the Gospel. The resemblance and agreement between the two are so great and so consistent as to establish, to the satisfaction of most minds, an identity of authorship.
Of these resemblances, the most obvious are certain verbal correspondences of language, of which the following examples will repay comparison. (1) Characteristic words used in a peculiar sense: 'life' (1 John 1:1; 1 John 3:14 cp. John 1:4; John 6:33; John 6:51); 'light' (1 John 1:5; 1 John 1:7; 1 John 2:8 cp. John 1:4; John 1:7; John 1:9); 'darkness' (1 John 1:6; 1 John 2:11 cp. John 8:12; John 12:35); 'world' (1 John 2:15; 1 John 4:4 cp. John 1:10; John 12:31; John 14:17). (2) Characteristic expressions: 'eternal life' (1 John 1:2; 1 John 3:15 cp. John 3:15; John 6:40; John 17:3); 'a new commandment' (1 John 2:8 cp. John 13:34); 'only begotten Son' (1 John 4:9 cp. John 1:18; John 3:16); 'know God' (1 John 2:3; 1 John 4:6 cp. John 17:3; John 17:25); 'abide in Christ' (1 John 2:6; 1 John 3:24 cp. John 6:56; John 15:4; John 15:5.) (3) Identical phrases; 'that your joy may befull' (1 John 1:4 cp. John 16:24); 'walketh in darkness, and knoweth not whither he goeth' (1 John 2:11 cp. John 12:35); (are) 'passed from death unto life' (1 John 3:14 cp. John 5:24); 'know him that is true' (1 John 5:20; (RV); cp. John 17:3). Besides these and other like examples, a general similarity of style and thought gives evidence almost the strongest of its kind to show that if St. John wrote the Gospel which bears his name, he wrote the Epistle also.
(b) External Evidence. The witness afforded by the book itself to its authorship is amply supported by the testimony of ancient writers. The Epistle is evidently quoted (though without mention of the fact) by Polycarp (116 a.d.), who was, according to Irenæus, a (disciple of St. John. It was used, Eusebius tells us, by Papias (120 a.d.), an associate of Polycarp, also said to have been a hearer of St. John. It is quoted and referred to as St. John's Epistle by Irenæus (180 a.d.), Polycarp's disciple, by Clement of Alexandria (190 a.d.), Tertullian (200 a.d.), Origen (230 a.d.), and others.