“Whosoever is born of God does not commit sin; for his seed remains in him: and he cannot sin, because he is born of God. In this the children of God are manifest, and the children of the devil: whosoever is not righteous is not of God, neither he that loves not his brother. For this is the message that you heard from the beginning, that we should love one another. Not as Cain, who was of the wicked one, and slew his brother. And wherefore slew he him? Because his own works were evil, and his brother's righteous. Marvel not, my brethren, if the world hate us. We know that we have passed from death unto life, because we love the brethren. He that loves not abides in death. Whosoever hates his brother is a murderer: and you know that no murderer has eternal life abiding in him. In this we know love, that He laid down His life for us: and we ought to lay down our lives for the brethren. But whoso has this world's good, and sees his brother have need, and shuts up his bowels of compassion from him, how can the love of God dwell in him? My little children, let us not love only in word and in tongue; but in deed and in truth.”
Source: Homilies on the First Epistle of John (New Advent)