John Calvin Commentary 1 John 4:19

"We love, because he first loved us." — 1 John 4:19 (ASV)

We love him. The verb ἀγαπῶμεν may be in either the indicative or imperative mood, but the former is the more suitable here. For the Apostle, as I think, repeats the preceding sentence: that as God has anticipated us with His free love, we ought to love Him in return, because the Apostle immediately infers that God ought to be loved in men, or that the love we have for God ought to be manifested towards men. If, however, the imperative mood is preferred, the meaning would be nearly the same: that as God has freely loved us, we also ought to love Him now.

But this love cannot exist unless it generates brotherly love. Hence, he says that those who boast that they love God, while hating their brothers, are liars.

But the reason he adds does not seem sufficiently valid, for it is a comparison between the lesser and the greater: If, he says, we do not love our brothers whom we see, much less can we love God who is invisible. Now, there are two obvious exceptions. First, the love God has for us is from faith and does not flow from sight, as we find in 1 Peter 1:8. Second, the love of God is far different from the love of men; for while God leads His people to love Him through His infinite goodness, men are often worthy of hatred. To this I answer that the Apostle here takes as granted what should undoubtedly be evident to us: that God offers Himself to us in those men who bear His image, and that He requires the duties, which He Himself does not need, to be performed for them, according to Psalm 16:2, where we read:

“My goodness reaches not to thee, O Lord;
towards the saints who are on the earth is my love.”

And surely, sharing the same nature, the need for so many things, and mutual interaction, must draw us to mutual love, unless we are harder than iron. But John meant another thing: he meant to show how fallacious is the boast of everyone who says that he loves God, and yet does not love God’s image which is before his eyes.