Early Christian tradition identified this work as a letter of John the apostle. Because of its resemblance to the fourth gospel in style, vocabulary, and ideas, it is generally agreed that both works are the product of the same school of Johannine Christianity. The terminology and the presence or absence of certain theological ideas in 1 John suggest that it was written after the gospel; it may have been composed as a short treatise on ideas that were developed more fully in the fourth gospel. To others, the evidence suggests that 1 John was written after the fourth gospel as part of a debate on the proper interpretation of that gospel. Whatever its relation to the gospel, 1 John may be dated toward the end of the first century. Unlike 2 and 3 John, it lacks in form the salutation and epistolary conclusion of a letter. These features, its prologue, and its emphasis on doctrinal teaching make it more akin to a theological treatise than to most other New Testament letters.

The purpose of the letter is to combat certain false ideas, especially about Jesus, and to deepen the spiritual and social awareness of the Christian community (1 Jn 3:17). Some former members (1 Jn 2:19) of the community refused to acknowledge Jesus as the Christ (1 Jn 2:22) and denied that he was a true man (1 Jn 4:2). The specific heresy described in this letter cannot be identified exactly, but it is a form of docetism or gnosticism; the former doctrine denied the humanity of Christ to insure that his divinity was untainted, and the latter viewed the appearance of Christ as a mere stepping-stone to higher knowledge of God. These theological errors are rejected by an appeal to the reality and continuity of the apostolic witness to Jesus. The author affirms that authentic Christian love, ethics, and faith take place only within the historical revelation and sacrifice of Jesus Christ. The fullness of Christian life as fellowship with the Father must be based on true belief and result in charitable living; knowledge of God and love for one another are inseparable, and error in one area inevitably affects the other. Although the author recognizes that Christian doctrine presents intangible mysteries of faith about Christ, he insists that the concrete Christian life brings to light the deeper realities of the gospel.

The structure and language of the letter are straightforward yet repetitious. The author sets forth the striking contrasts between light and darkness, Christians and the world, and truth and error to illustrate the threats and responsibilities of Christian life. The result is not one of theological argument but one of intense religious conviction expressed in simple truths. The letter is of particular value for its declaration of the humanity and divinity of Christ as an apostolic teaching and for its development of the intrinsic connection between Christian moral conduct and Christian doctrine.

The principal divisions of the First Letter of John are the following:

1. I. Prologue (1:1–4)
2. II. God as Light (1:5–3:10)
3. III. Love for One Another (3:11–5:12)
4. IV. Epilogue (5:13–21)

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