1-john 5:2

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.

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.

5:2 Hereby we know - This is a plain proof. That we love the children of God - As his children.