Numbers 3:12

And I, behold, I have taken the Levites from among the
children of Israel
Separated them from others, and set them apart for the service of the sanctuary; this was his own act and deed, and which he did of his own will and pleasure, who is a sovereign Being, and might and would do whatsoever he pleased, nor should any object unto him, or contradict him: and this he did,

instead of all the firstborn that openeth the matrix among the
children of Israel;
he made an exchange of those for the Levites; upon the destruction of the firstborn of Egypt, and saving the firstborn of Israel, he claimed the latter as his own in a special sense, and now he gave up his right to them, and instead thereof took the Levites; nor could the people of Israel reasonably object to this, nor be uneasy at it, but rather be pleased with it; since hereby they were not only freed from the charge of redeeming their firstborn, but since they were the Lord's, he might have appointed them to be servants to the priests; and every Israelite would choose rather, no doubt, to part with a tribe for this service than to have their firstborn sons employed in it; and there were none so fit as the tribe of Levi, not only because it was a small tribe, but because the priests were of this tribe, to whom they were to minister; and therefore as there was a connection between them, the Levites would readily serve them: it is a notion that has obtained both among Jews and Christians, that the firstborn before this time were priests in the family, but now the Levites were taken in their room; and Jarchi particularly observes, that when the firstborn sinned in the business of the calf, they were rejected, and the Levites, who did not commit idolatry, were chosen in their room; but it does not appear, before the fixing the priesthood in Aaron's family, that the firstborn in a family were priests, and officiated as such, but rather the father and head of the family for the whole, or everyone for himself, as in Adam's family, he and his two sons; and moreover, it was a recent thing, and only among the Hebrews, that the firstborn were in a peculiar manner the Lord's, not as priests, only to be redeemed, and now the Levites were redeemed in their room; it was not to be priests, but the servants of the priests ^F18:

therefore the Levites shall be mine;
whom he gave to the priests to assist them in their work, which was for him and his glory.