Albert Barnes Commentary 1 John 5:2

"Hereby we know that we love the children of God, when we love God and do his commandments." — 1 John 5:2 (ASV)

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 will 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 says here 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. There can be no doubt that we may have evidence that we love God, apart from the evidence we derive from our love to his children.

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 by itself than it is to determine whether we love God? (Compare to 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, sect, or party; or we may love them because they are naturally amiable. But the apostle says here that when we are conscious that an attachment exists 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 will then love them as his children, whatever other grounds of affection there may be towards them.

And keep his commandments. See Barnes on John 14:16.