1 John 4:20

But this love cannot exist, except it generates brotherly love. Hence he says, that they are liars who boast that they love God, when they hate their brethren.

But the reason he subjoins seems not sufficiently valid, for it is a comparison between the less and the greater: If, he says, we love not our brethren whom we see, much less can we love God who is invisible. Now there are obviously two exceptions; for the love which God has to us is from faith and does not flow from sight, as we find in 1 Peter 1:8; and secondly, far different is the love of God 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 takes here as granted what ought no doubt to appear evident to us, that God offers himself to us in those men who bear his image, and that he requires the duties, which he does not want himself, to be performed to them, according to Psalms 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 the participation of the same nature, the need of so many things, and mutual intercourse, must allure us to mutual love, except; we are harder than iron. But John meant another thing: he meant to shew how fallacious is the boast of every one who says that he loves God, and yet loves not God’s image which is before his eyes.