Jamieson, Fausset and Brown on 1 John 5:2

2. By--Greek, "In." As our love to the brethren is the sign and test of our love to God, so (John here says) our love to God (tested by our "keeping his commandments") is, conversely, the ground and only true basis of love to our brother.

we know--John means here, not the outward criteria of genuine brotherly love, but the inward spiritual criteria of it, consciousness of love to God manifested in a hearty keeping of His commandments. When we have this inwardly and outwardly confirmed love to God, we can know assuredly that we truly love the children of God. "Love to one's brother is prior, according to the order of nature (see on 1Jo 4:20); love to God is so, according to the order of grace (1Jo 5:2). At one time the former is more immediately known, at another time the latter, according as the mind is more engaged in human relations or in what concerns the divine honor" [Estius]. John shows what true love is, namely, that which is referred to God as its first object. As previously John urged the effect, so now he urges the cause. For he wishes mutual love to be so cultivated among us, as that God should always be placed first [Calvin].