It the fourth of the catholic or "general" epistles. It was traditionally held to have been written by John the Evangelist, and probably also at Ephesus, and when the writer was in advanced age.
Although the epistle's content shows much agreement with the Gospel of John (without, however, quoting that book), it is today an open question whether a common authorship can be assumed.
The purpose of the apostle (1:1-4) is to declare the Word of Life to those to whom he writes, in order that they might be united in fellowship with the Father and his Son Jesus Christ. He shows that the means of union with God are, (1) on the part of Christ, his atoning work (1:7; 2:2; 3:5; 4:10, 14; 5:11, 12) and his advocacy (2:1); and (2), on the part of man, holiness (1:6), obedience (2:3), purity (3:3), faith (3:23; 4:3; 5:5), and love (2:7, 8; 3:14; 4:7; 5:1).