Charles Ellicott Commentary Numbers 3:41

"And thou shalt take the Levites for me (I am Jehovah) instead of all the first-born among the children of Israel; and the cattle of the Levites instead of all the firstlings among the cattle of the children of Israel:" — Numbers 3:41 (ASV)

And you shall take the Levites for me (I am the Lord) ... — Or, And you shall take the Levites for Me—for Me, Jehovah. The assertions which have been frequently made concerning the transfer of the priesthood of the firstborn to the Levites appear to be altogether without foundation. For:

1. The priesthood that was exercised in patriarchal times was not restricted to the firstborn, but appears to have been common to the head of every family.
2. This priesthood was exercised before the exodus, and consequently before the command given to Moses to sanctify the firstborn.
3. The priesthood, which belonged not to the firstborn exclusively, but to the Israelites at large, was thereafter strictly confined to the family of Aaron, who inherited it not as the substitutes of the firstborn, but in place of the whole nation.