"God is love" is a phrase that extends far beyond the boundaries of theology and the church, characterizing the Christian discourse about God. In the New Testament, it appears only in the First Letter of John. From this, the task of the present commentary arises to interpret the writing resolutely as an introduction to the reality of love: divine love, which has found its unparalleled clarity in Jesus Christ, and human love, which must prove itself tangibly and concretely in fraternal interactions. The First Letter of John connects love to confession and thus to faith. The dynamic relationship between faith and love constitutes the actual theological core of the letter, which the explanation seeks to elucidate. The basis for the interpretation consists of in-depth analyses of philological, structural, and literary nature. The introduction provides orientation regarding the situation of the community, which faced the painful experience of a schism, and the historical-religious context of this controversy. Excursions address topics such as the love for sisters and brothers and the question of what is called early Catholicism. Special emphasis is placed on the history of interpretation and impact. Thus, exegesis is in constant dialogue with Augustine and Martin Luther. Guided by the hope that fruitful ecumenical perspectives may emerge from this, certain lines are drawn into patristic theology, the history of dogmas and piety, systematic theology, and philosophy. The complementary, carefully reflected steps of work reveal both the greatness and limitations of a fascinating theological design that holds perspectives of surprising relevance.