This law restrains men from disinheriting their eldest sons out of mere caprice, and without just provocation.
I. The case here put (Deu 21:15) is very instructive. 1. It shows the great mischief of having more wives than one, which the law of Moses did not restrain, probably in hopes that men's own experience of the great inconvenience of it in families would at last put an end to it and make them a law to themselves. Observe the supposition here: If a man have two wives, it is a thousand to one but one of them is beloved and the other hated (that is, manifestly loved less) as Leah was by Jacob, and the effect of this cannot but be strifes and jealousies, envy, confusion, and every evil work, which could not but create a constant uneasiness and vexation to the husband, and involve him both in sin and trouble. Those do much better consult their own ease and satisfaction who adhere to God's law than those who indulge their own lusts. 2. It shows how Providence commonly sides with the weakest, and gives more abundant honour to that part which lacked; for the first-born son is here supposed to be hers that was hated; it was so in Jacob's family: because the Lord saw that Leah was hated, Gen 29:31. The great householder wisely gives to each his dividend of comfort; if one had the honour to be the beloved wife, it often proved that the other had the honour to be the mother of the first-born.
II. The law in this case is still binding on parents; they must give their children their right without partiality. In the case supposed, the eldest son, though the son of the less-beloved wife, must have his birthright privilege, which was a double portion of the father's estate, because he was the beginning of his strength that is, in him his family began to be strengthened and his quiver began to be filled with the arrows of a mighty man (Psa 127:4), and therefore the right of the first-born is his, Deu 21:16, Deu 21:17. Jacob had indeed deprived Reuben of his birthright, and given it to Joseph, but it was because Reuben had forfeited the birthright by his incest, not because he was the son of the hated; now, lest that which Jacob did justly should be drawn into a precedent for others to do the same thing unjustly, it is here provided that when the father makes his will, or otherwise settled his estate, the child shall not fare the worse for the mother's unhappiness in having less of her husband's love, for that was not the child's fault. Note, (1.) Parents ought to make no other difference in dispensing their affections among their children than what they see plainly God makes in dispensing his grace among them. (2.) Since it is the providence of God that makes heirs, the disposal of providence in that matter must be acquiesced in and not opposed. No son should be abandoned by his father till he manifestly appear to be abandoned of God, which is hard to say of any while there is life.

Then it shall be, when he maketh his sons to inherit that which he hath,.... By a will in writing, or byword of mouth, or by a deed of gift, actually bestowing his goods upon them, and dividing among them what he is for the present possessed of; see Luk 15:12,

that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn; that is, when such is the case, that the son of his wife he has the least value for is really his firstborn, he may not, through favour and affection to the wife he loves better, prefer her son, and declare him to be the firstborn, by devising to him or bestowing on him the double portion of his goods; for so to do would not be right, or agreeably to the will and law of God; for though previous to this law the birthright was given to Joseph, the eldest son of Rachel, the most beloved wife of Jacob, before Reuben who was the son of Leah, less beloved by him, and was in fact his firstborn; yet this was owing to the sin of Reuben, and by the appointment of God; see Gen 49:3.

If a man have two wives, one beloved, and another hated--In the original and all other translations, the words are rendered "have had," referring to events that have already taken place; and that the "had" has, by some mistake, been omitted in our version, seems highly probable from the other verbs being in the past tense--"hers that was hated," not "hers that is hated"; evidently intimating that she (the first wife) was dead at the time referred to. Moses, therefore, does not here legislate upon the case of a man who has two wives at the same time, but on that of a man who has married twice in succession, the second wife after the decease of the first; and there was an obvious necessity for legislation in these circumstances; for the first wife, who was hated, was dead, and the second wife, the favorite, was alive; and with the feelings of a stepmother, she would urge her husband to make her own son the heir. This case has no bearing upon polygamy, which there is no evidence that the Mosaic code legalized.