John Calvin Commentary Genesis 42:15

"hereby ye shall be proved: by the life of Pharaoh ye shall not go forth hence, except your youngest brother come hither." — Genesis 42:15 (ASV)

By the life of Pharaoh. From this formula of swearing, a new question is raised. For what is commanded in the law—that we should swear only by the name of God—had already been engraved on the hearts of the pious, since nature dictates that this honor is to be given to God alone, namely, that people should defer to His judgment and make Him the supreme arbiter and vindicator of faith and truth.

If we were to say that this was not simply an oath, but a kind of solemn pledge, the holy man would be, to some degree, excusable. One who swears by God wishes Him to intervene to inflict punishment on perjury. Those who swear by their life or by their hand deposit, as it were, what they consider most valuable as a pledge of their faithfulness.

By this method, the majesty of God is not transferred to a mortal man, because it is a very different thing to cite God as a witness who has the right to take vengeance, and to assert by something most dear to us that what we say is true. So Moses, when he calls heaven and earth to witness, does not ascribe deity to them and thus fabricate a new idol. Instead, so that higher authority may be given to the law, he declares that there is no part of the world which will not cry out before the tribunal of God against the ingratitude of the people if they reject the doctrine of salvation.

Notwithstanding, I confess, there is something in this form of swearing which Joseph uses that is deserving of censure. For it was a profane adulation among the Egyptians to swear by the life of the king, just as the Romans swore by the genius of their prince after they had been reduced to such bondage that they made their Caesar equal to gods.

Certainly, this mode of swearing is abhorrent to true piety. From this it may be perceived that nothing is more difficult for the holy servants of God than to keep themselves so pure, while living amidst the filth of the world, as not to contract any spots of defilement from it. Joseph, indeed, was never so infected with the corruptions of the court that he ceased to be a pure worshipper of God; nevertheless, we see that in accommodating himself to this depraved custom of speaking, he had received some stain.

His repetition of the expression shows that when anyone has once become accustomed to evil, they become exceedingly prone to sin repeatedly. We observe that those who have once rashly assumed the license of swearing pour forth an oath every third word, even when speaking of the most frivolous things. Therefore, we ought to use all the more caution, lest any such indulgence harden us in this wicked custom.