By this we know that we love the children of God, when we love God, and keep his commandments.
By - `IN this.' As love to the brethren is the test of our love to God, so love to God (tested by 'keeping His commandments') is, conversely, the only basis of real love to our brother.
We know. John means not the outward criteria of genuine brotherly love, but the inward criteria, consciousness of love to God, manifested in heartily keeping His commandments. When we have this inwardly-and-outwardly-confirmed love to God, we know that we truly love the children of God. 'Love to one's brother is prior in the order of nature (note, 1 John 4:20); love to God is so in the order of grace (1 John 5:2). At one time the former is more known, at another the latter, according as the mind is more occupied with human relations, or with what concerns the divine honour' (Estius). True love is referred to God as its first object. Previously John urged the effect; now he urges the cause. For he wishes us so to cultivate mutual love, as that God should always be placed first (Calvin).