1 John 5:2

"In this we know that we love the sons of God, in that we love God." And how? Are not the sons of God one thing, God Himself another? But he that loves God, loves His precepts. And what are the precepts of God? "A new commandment give I unto you, that ye love one another." Let none excuse himself by another love, for another love; so and so only is it with this love: as the love itself is compacted in one, so all that hang by it doth it make one, and as fire melts them down into one. It is gold: the lump is molten and becomes some one thing. But unless the fervor of charity be applied, of many there can be no melting down into one. "That we love God," by this "know we that we love the sons of God."

In this we know that we love the children of God, etc. Therefore, he alone is proven to love his neighbor rightly who is known to burn with the love of the Creator. And lest anyone deceive himself about the love of the Creator, professing …

And this is said in confirmation of the above. If we received from the Teacher the commandment to love one another, then, without a doubt, if we believe that our Teacher Jesus is the Christ, and He is the Christ in the sense that He …

Our love to them appears to be sound and genuine when we love them not merely upon any secular account, as because they are rich, or learned, or kind to us, or of our denomination among religious parties; but because they are God's children, his regenerating grace appears in them, his image and superscription are upon them, and so in them God himself is loved.

By this we know that we are the children 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].

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, 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 easier to ascertain this of itself than it is to determine whether we love God? Compare 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 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 Joh 14:15.