Writer: The Apostle John, as unbroken tradition affirms, and as internal evidence and comparison with the Gospel of John prove.
Date: Probably A.D. 90.
Theme: First John is described as a family letter from the Father to His ‘little children’ who are in the world. The “world” (kosmos) is seen as outside the family. The believer’s sin is treated not as a question of the universe’s moral government, but as a child’s offence against his Father, and that matter has already been dealt with in the Cross. The Lord Jesus Christ now acts as an “advocate with the Father.”
Purpose: John’s Gospel brings the reader across the threshold of the Father’s house; his first Epistle makes us at home there. John uses the tender term teknia (“little children”, “born-ones”) rather than concentrating on public position as sons (as Paul often does), emphasising nearness to the Father in the family.
Structure: The epistle divides into two main sections:

1. The family with the Father (1 John 1:1–3:24)
2. The family and the world (1 John 4:1–5:21)

Summary: In short, this epistle is not primarily a theological treatise on justification or eschatology (though these themes appear), but a pastoral, intimate letter: from the Father to His children, emphasising fellowship with the Father and the Son, genuine living in the fellowship, and distinguishing the believer’s family-relationship from the world outside. The moral and doctrinal challenges are addressed in the context of the believer’s nearness to the Father, not simply as subjects under law.