John Calvin Commentary 1 John 3:12

"not as Cain was of the evil one, and slew his brother. And wherefore slew he him? Because his works were evil, and his brother`s righteous." — 1 John 3:12 (ASV)

Not as Cain. This is another confirmation, taken from its opposite, for in the reprobate and the children of the devil, hatred reigns and holds, as it were, the chief place in their life. He brings forward Cain as an example. This served, in the meantime, to give them consolation, as he finally concluded by saying, Marvel not, if the world hate you.

This explanation should be carefully noted, for people continually blunder regarding the way of living because they believe holiness consists of fictitious works. While they torment themselves with trifles, they think themselves doubly acceptable to God, like the monks who proudly call their way of life a state of perfection; nor is there any other worship of God under the Papacy than a mass of superstitions.

But the Apostle testifies that this righteousness alone is approved by God—that is, if we love one another. He further testifies that the devil reigns where hatred, dissimulation, envy, and enmity prevail.

We should, however, at the same time, remember what I have already mentioned: that brotherly love, proceeding from the love of God as an effect from its cause, is not separated from it. On the contrary, John commends it for this reason: because it is evidence of our love for 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 so that we may learn to endure patiently when the world hates us without cause, without any just provocation.