John Calvin's Commentary on the Bible

12 (1 John 3:12)

Verse 12

12Not 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.

- John Calvin's Commentary on the Bible