Hereby we know that we love the children of God, when we love God and do his commandments.

in this we know that we love the children of God, when we may love God, and His commands may keep;

The fact that we love God Himself, and obey His commands, is a proof that we love God's children.

In this way, we are certain that we have love for the children of God, when we have love for God and keep his laws.

In this way, we know that we love those born of God: when we love God and do his commandments.

By this we know that we love God's children – when we love God and carry out his commands.

By this we know He briefly shows in these words what true love is, even that which is towards God. He has hitherto taught us that there is never a true love to God, except when our brethren are also loved; for this is ever its effect. But he now teaches us that men are rightly and duly loved, when God holds the primacy. And it is a necessary definition; for it often happens, that we love men apart from God, as unholy and carnal friendships regard only private advantages or some other vanishing objects. As, then, he had referred first to the effect, so he now refers to the cause; for his purpose is to shew that mutual love ought to be in such a way cultivated that God may be honored. To the love of God he joins the keeping of the law, and justly so; for when we love God as our Father and Lord, reverence must necessarily be connected with love. Besides, God cannot be separated from himself. As, then, he is the fountain of all righteousness and equity, he who loves him must necessarily have his heart prepared to render obedience to righteousness. The love of God, then, is not idle or inactive. [1] But from this passage we also learn what is the keeping of the law. For if, when constrained only by fear, we obey God by keeping his commandments, we are very far off from true obedience. Then, the first thing is, that our hearts should be devoted to God in willing reverence, and then, that our life should be formed according to the rule of the law. This is what Moses meant when, in giving a summary of the law, he said, "O Israel, what does the Lord thy God require of thee, but to love him and to obey him?" (Deuteronomy 10:12.)

Footnotes

1 - The love of God," here clearly means love to God: it is the love of which God is the object. -- Ed.

By this we know that we love the children of God - 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, 1-John 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 easier to ascertain this of itself than it is to determine whether we love God? Compare 1-John 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 toward 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 toward them.
And keep his commandments - See the notes at John 14:15.

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.

(2) By this we know that we love the children of God, when we love God, and keep his (c) commandments.
(2) The love of our neighbour depends on the love with which we love God, that this last must go before the first: of which it follows, that that is not to be called love, when men agree together to do evil, neither that, when as in loving our neighbours, we do not respect God's commandments.
(c) There is no love where there is no true doctrine.

By this we know that we are the children God,.... The Ethiopic version reads, "by this know that we love God"; which, in connection with what follows, makes a tautology, and is a proving "idem per idem": whereas the apostle's view is to show when love to the saints is right; and that is,
when we love God, and keep his commandments: love to the brethren may arise from such a cause, as may show that it is not brotherly love, or of a spiritual kind; it may arise from natural relation, or civil friendship, or from a benefit or favour received from them, and from some natural external excellency seen in them; and a man may do acts of love and kindness to the brethren, from what may be called good nature in himself, or with sinister views; but true love to the brethren springs from love to God: such who love the saints aright, and by which they may know they do so, they love them because they themselves love God, and in obedience to his command; they love them because they belong to God, and are the objects of his love; because his grace is wrought in them, and his image stamped upon them.

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 1-John 4:20); love to God is so, according to the order of grace (1-John 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].

Hereby we know - This is a plain proof. That we love the children of God - As his children.