John Calvin Commentary 1 John 4:16

"And we know and have believed the love which God hath in us. God is love; and he that abideth in love abideth in God, and God abideth in him." — 1 John 4:16 (ASV)

And we have known and believed: it is as if he had said, “We have known by believing,” for such knowledge is not attained except by faith. From this we learn how different an uncertain or doubtful opinion is from faith. Besides, though he meant here, as I have already said, to accommodate the last sentence to his readers, he still defines faith in various ways. He had said before that it is to confess that Jesus is the Son of God; but he now says, we know by faith God’s love towards us. Thus it appears that the paternal love of God is found in Christ, and that nothing certain is known of Christ, except by those who know themselves to be the children of God by his grace. For the Father sets his own Son daily before us for this purpose, that he may adopt us in him.

God is love: this is, as it were, the minor proposition in an argument; for from faith to love he reasons in this way: By faith God dwells in us, and God is love; then, wherever God abides, love ought to be there. Thus it follows that love is necessarily connected with faith.