1 John 5:1,2

A reiteration of the doctrine that love for God = love for the brethren. Where either is, the other is also. Love for God is the inner principle, love for the brethren its outward manifestation. The argument is “an irregular Sorites” (Plummer):

Every one that hath faith in the Incarnation is a child of God;

Every child of God loves the Father;

... every one that hath faith in the Incarnation loves God.

Every one that hath faith in the Incarnation loves God;

Every one that loves God loves the children of God;

... every one that hath faith in the Incarnation loves the children of God.

These are the two commandments of God, the fundamental and all-embracing Christian duties love God and love the brotherhood. And faith in the Incarnation (ὅτι Ἰησοῦς ἐστιν ὁ Χριστός) is an inspiration for both.

πιστεύων corresponds to πίστις (1 John 5:4). The lack of a similar correspondence in English is felt here as in many other passages (e.g., Matthew 8:10; Matthew 8:13; Matthew 9:28-29). Latin is similarly defective: “omnis qui credit,” “ fides nostra”.