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].

Robert Jamieson, A. R. Fausset & David Brown, 1871 · public domain

Verse 2. By this we know that we love the children of God, etc. This is repeating the same truth in another form. "As it is universally true that if we love him who has begotten us, we shall also love his children, or our Christian brethren, so it is true also that if we love his children it will follow that we love him." In other places, the apostle says that we may know that we love God if we love those who bear his image, 1Jn 3:14. He here says, that there is another way of determining what we are. We may have undoubted evidence that we love God, and from that, as the basis of an argument, we may infer that we have true love to his children. Of the fact that we may have evidence that we love God, apart from that which we derive from our love to his children, there can be no doubt. We may be conscious of it; we may find pleasure in meditating on his perfections; we may feel sure that we are moved to obey him by true attachment to him, as a child may in reference to a father. But, it may be asked, how can it be inferred from this that we truly love his children? Is it not more easy to ascertain this of itself than it ia to determine whether we love God? Comp. 1Jn 4:20. To this it may be answered, that we may love Christians from many motives: we may love them as personal friends; we may love them because they belong to our church, or sect, or party; we may love them because they are naturally amiable: but the apostle says here, that when we are conscious that an attachment does exist towards Christians, we may ascertain that it is genuine, or that it does not proceed from any improper motive, by the fact that we love God. We shall then love him as his children, whatever other grounds of affection there may be towards them.

And keep his commandments. Jn 14:16

Albert Barnes, 1834 · public domain

Verse 2 By this we know that we love the children of God - Our love of God's followers is a proof that we love God. Our love to God is the cause why we love his children, and our keeping the commandments of God is the proof that we love him.

Adam Clarke, 1831 · public domain