1 John

The Book of 1 John seems to be a summary that assumes the readers’ knowledge of the gospel as written by John and offers certainty for their faith in Christ. The first epistle indicates that the readers were confronted with the error of gnosticism, which became a more serious problem in the second century. As a philosophy of religion it held that matter is evil and spirit is good. The solution to the tension between these two was knowledge, or gnosis, through which man rose from the mundane to the spiritual. In the gospel message, this led to two false theories concerning the person of Christ, Docetism—regarding the human Jesus as a ghost—and Cerinthianism—making Jesus a dual personality, at times human and at times divine. The key purpose of 1 John is to set boundaries on the content of faith and to give believers assurance of their salvation.

Reference: https://www.gotquestions.org/Book-of-1-John.html