Deuteronomy 21:17

But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his.

But he shall acknowledge the son of the hated for the firstborn,.... Own him and declare him to be so, both by his will and the division of goods by him; or he shall "separate" him, as Onkelos; distinguish him from all his other sons, and make known to all, as the Targum of Jonathan, that he is his firstborn:

by giving him a double portion of all that he hath; or, "that is found with him" {t}; which he was in the possession of when he made his will, or divided his goods; and so refers not to what might come into his hands afterwards, or should be his in reversion afterwards; in this the firstborn had not his double portion, only in what his father was for the present possessed of; so that if a man had two sons, his goods were divided into three parts, and the firstborn took two parts, and the other the third; if three sons, they were divided into four parts, of which the firstborn had two parts, and the others each of them one; if four sons, they were divided into five parts, and the firstborn took two, and the other three one apiece, and so in proportion; the division was made according to their number:

for he is the beginning of his strength; as Jacob said, of Reuben,

See Gill on "Genesis 49:3" the right of the firstborn is his; before this law was given, there was a birthright, or a privilege belonging to the firstborn, which gave him the preeminence in the family to his brethren; but whether he was entitled to a double portion of goods, previous to this law, is not certain; however, by this it was his right, and might not be alienated from him; for, according to the Jewish canons {u},

"if a man say, such an one my son, the firstborn, shall not take the double portion, and my son such an one shall not inherit with his brethren, he says nothing, cause he disposes contrary to what is written in the law.''

This law of the firstborn in the mystery of it may respect our Lord Jesus Christ, the firstborn of God, and the firstborn of Mary; and who had a double portion of the gifts and grace of the Spirit, or rather the Spirit without measure, the oil of gladness he was anointed with above his fellows, and is the firstborn among many brethren, among whom in all things he has the preeminence; and also the elect of God, the church of the firstborn, whose names are written in heaven, who have a double portion, both temporal and spiritual things, the promise of this life and that to come, grace here and glory hereafter; and the ultimate glory is but one inheritance, they all share alike in, being equally children, and all firstborn; and it may have regard also to the Jewish and Gentile churches, the former was the beloved wife, the latter some time not beloved, and yet the children of the Gentile church have a larger measure of the Spirit than those of the jewish church had; see Romans 9:25.

{t} wl aumy rva "quod inventum fuerit ei", Pagninus, Montanus.,
{u} Misn. Bava Bathra, c. 8. sect. 5.