John Calvin Commentary Deuteronomy 21:15

"If a man have two wives, the one beloved, and the other hated, and they have borne him children, both the beloved and the hated; and if the first-born son be hers that was hated;" — Deuteronomy 21:15 (ASV)

If a man has two wives. Since it is here provided that a father should not unjustly transfer what belongs to one son to another, this is a part and supplement of the Eighth Commandment, the substance of which is that everyone’s rights should be preserved for them. For, if the father substituted another son in the place of his firstborn, it was unquestionably a kind of theft.

But, since it rarely happens that a father unnaturally degrades his firstborn from his precedence if all are born of the same mother, God reminds us that He did not enact this law without cause. For, where polygamy was allowed, the husband's mind was generally most inclined to the second wife. If he had loved the first with true affection, he would have been contented with her as the companion of his life and bed, and would not have thought of a second.

When, therefore, the husband grew tired of his first wife and desired a second, he might be coaxed by her flattery to withhold from the children of his first marriage what naturally belonged to them. Therefore, this remedy was necessary, by which the father’s power of altering the right of primogeniture is barred.

For, although they might allege that they only gave what was their own, yet it was an act of ungodly arrogance to reject him whom God had deigned to honor. For he who arrogates such power to himself, or who assigns the birthright to whomever he pleases, almost arrogates to himself the ability to create.

This right, as is stated in Deuteronomy 21:17, was a double portion of the paternal inheritance. The reason which is added is equivalent to saying that the firstborn is the principal honor and ornament of the father.

Still, if there was a just cause for disinheriting the firstborn, another successor might be substituted in his place, as Jacob showed in his case when he disinherited Reuben (Genesis 49:4).

When it is said, before the son of the hated, some interpret it to mean “during his lifetime;” others retain the Hebrew phrase, “before his face.” However, the opinion of those who take this particle comparatively, meaning “instead of her son,” is probable.

The wife is called hated, not because her husband is actually her enemy, but because he loves her least. For contempt is considered as hatred, and he is called an enemy who does not provide marital affection.