Numbers 3:12

“And I, behold, I have taken the Levites from among the children of Israel, etc.] 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…”

“The first - born - Who were God's property, Exo 13:12, and to whom the administration of holy things was formerly committed, which now was taken away from them, either because they had forfeited this privilege by joining with the rest of their brethren in the idolatrous worship of the calf, or because they were to be mainly concerned in the distribution and management of the inheritances which now they were going to possess, and therefore could not be at leisure to attend upon the service of the sanctuary: and God would not commit it to some other persons in each tribe, which might be an occasion of idolatry, confusion, division, and contempt of sacred things, but to one distinct tribe,…”

“When God miraculously destroyed all the first-born of the Egyptians ( Exo 12:29), he spared those of the Israelites; and, in commemoration of that event, he was pleased to appoint that all the first-born males "should be set apart unto himself" ( Exo 13:12-16). God is here pleased to relinquish this claim, and to appoint the whole tribe of Levi to attend his immediate service in their stead. Num 3:41, Num 3:45, Num 8:16, Num 8:18, Num 18:6 Numbers 3:13”