John Calvin on 1 John 3:12-13

12 Not as Cain This is another confirmation, taken from what is contrary; for in the reprobate and the children of the devil hatred reigns, and it holds, as it were, the chief place in their life; and he brings forward Cain as an instance. It served in the meantime to give them consolation, as he at length concluded by saying, Marvel not, if the world hate you.

This explanation ought to be carefully noticed, for men ever blunder as to the way of living, because they make holiness to consist of fictitious works, and while they torment themselves with trifles, they think themselves doubly acceptable to God, as the monks, who proudly call their mode of living a state of perfection; nor is there any other worship of God under the Papacy but a mass of superstitions. But the Apostle testifies that this righteousness alone is approved by God, that is, if we love one another; and further, that the devil reigns where hatred, dissimulation, envy, and enmity prevail. We ought, however, at the same time, to bear in mind what I have already touched upon, that brotherly love, as it proceeds from the love of God as an effect from a cause, is not disjoined from it, but on the contrary is commended by John on this account, because it is an evidence of our love to God.

By saying that Cain was driven to slay his brother, because his works were evil, he intimates what I have already stated, that when impiety rules, hatred occupies the first place. He refers to Abel’s righteous works, that we may learn to endure patiently when the world hates us gratuitously, without any just provocation.

1 John 3:15-18

14. We know that we have passed from death unto life, because we love the brethren. He that loveth not his brother abideth in death.

14. Nos scimus quod transierimus a morte in vitam, quia diligimus fratres: qui non diligit fratrem, manet in morte.

15. Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.

15. Omnis qui odit fratrem suum, homicida est; et nostis quod omnis homicida, non habet vitam aeternam in se manentem.

16. Hereby perceive we the love of God, because he laid down his life for us: and we ought to lay down our lives for the brethren.

16. In hoc cognoscimus charitatem, quod ille pro nobis animam suam posuit: et nos debemus pro fratribus animas ponere.

17. But whoso hath this world’s good, and seeth his brother have need, and shutteth up his bowels of compassion from him, how dwelleth the love of God in him?

17. Si quis habeat victum mundi, et videat fratrem suum egentem, et claudat viscera sua ab eo, quomodo charitas Dei in ipso manet?

18. My little children, let us not love in word, neither in tongue; but in deed and in truth.

18. Filioli mei, ne diligamus sermone, neque lingua, sed opere et veritate.