From one of John Calvin’s sermons on Genesis, we read:
For faith has this property, that it confines us within divinely prescribed bounds, so that we attempt nothing except with God’s authority or permission. Whence it follows that Isaac’s faith wavered when he swerved from his duty as a husband. We gather, besides, from the words of Abimelech, that all nations have the sentiment impressed upon their minds, that the violation of holy wedlock is a crime worthy of divine vengeance, and have consequently a dread of the judgment of God. For although the minds of men are darkened with dense clouds, so that they are frequently deceived; yet God has caused some power of discrimination between right and wrong to remain, so that each should bear about with him his own condemnation, and that all should be without excuse. If, then, God cites even unbelievers to his tribunal, and does not suffer them to escape just condemnation, how horrible is that punishment which awaits us, if we endeavor to obliterate, by our own wickedness, that knowledge which God has engraven on our consciences? (comment. Gen. 26:10)
The typical features of Calvin’s philosophy of law are present here. He notes that while sin does affect the mind and the will, God never allows it to so obscure the innate knowledge of the good as to destroy basic morality. Calvin believes that “all nations” understand adultery to be a grave sin, thus affirming ius gentium. Individuals are always capable of suppressing or denying this moral knowledge, of course, and in this case, Abimelech’s righteousness serves as a judgment against Isaac.