10 “If the world hate us: we know”— What do we know?— “that we have passed from death unto life”— How do we know? “Because we love the brethren.” Let none ask man: let each return to his own heart: if he find there brotherly love, let him set his mind at rest, because he is “passed from death unto life.” Already he is on the right hand: let him not regard that at present his glory is hidden: when the Lord shall come, then shall he appear in glory. For he has life in him, but as yet in winter; the root is alive, but the branches, so to say, are dry: within is the substance that has the life in it, within are the leaves of trees, within are the fruits: but they wait for the summer.
Well then, “we know that we have passed from death unto life, because we love the brethren. He that loves not, abides in death.” Lest ye should think it a light matter, brethren, to hate, or, not to love, hear what follows: “Every one that hates his brother, is a murderer.” How now, if any made light of hating his brother, will he also in his heart make light of murder? He does not stir his hands to kill a man; yet he is already held by God a murderer; the other lives, and yet this man is already judged as his slayer! “Every one that hates his brother is a murderer: and you know that no murderer has eternal life abiding in him.”
Source: Homilies on the First Epistle of John (New Advent)