Exodus 13:2

Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast: it is mine.

Sanctify unto me all the firstborn,.... That is, of males, as the Targum of Jonathan adds, for those, and not females, were only either sacrificed or redeemed, see Exodus 13:12, and this sanctification of them to the Lord signifies the separation or devoting of them to the service of God; if the firstborn of clean creatures they were to be sacrificed, if unclean to be redeemed with a price, and so the firstborn of men, because it was not lawful to sacrifice them; and the money for the redemption of them was given to the priests, the ministers of the Lord, and so to him; who these first, born were is further explained:

whatsoever openeth the womb among the children of Israel, both of man and of beast; that is, if a male; for, if a female, though it openeth the womb, was not reckoned a firstborn, because not to be offered; nor even a male after the birth of a female, because that openeth not the womb; and so if a man married a widow, and she had had children by her former husband, though she should bring him a son, which was his firstborn, yet not being her's, and not opening the womb, was not subject to this law; but if a man married several wives one after another, or together, who had never been married before, or had had no children; if each of them brought him a son at first birth, they were all of them firstborn, and to be sanctified to the Lord; but the Jews say {u}, if a woman at her first birth brought forth a male and a female, the father was free from this law of the redemption of the firstborn, because the female might come forth first: this phrase, "among the children of Israel", shows that this law only belonged to them, and not to the Gentiles; wherefore the Jewish doctors say {w}, if a man buys cattle of an Heathen, and sells to him, or is in partnership with him, and gives and takes of him, he is free from the law of the firstborn; for it is said "among the Israelites", and not among others:

it is mine: all creatures, man and beast, are the Lord's by creation; but these firstborn were his in a peculiar manner, and which he reserved to himself, to his own use and service; and the people of Israel were under great obligation to devote them to him, since he had spared all their firstborn, when all the firstborn of the Egyptians, both man and beast, were destroyed: this may denote the special and peculiar interest the Lord has in the general assembly and church of the firstborn, whose names are written in heaven, through the special, particular, and eternal choice of them in Christ, and the redemption of them to him by the price of his blood; and who, on account both of their election of God, and redemption by Christ, are laid under obligation to give up themselves to God, a holy, living, and acceptable sacrifice, which is but their reasonable service.

{u} Misn. Becorot, c. 8. sect. 4, 5. & Bartenora in ib.
{w} lbid. c. 2. sect. 1.