4 But Nadab and Abihu died in the Lord 's presence in the wilderness of Sinai when they burned before the Lord the wrong kind of fire, different than he had commanded. Since they had no sons, this left only Eleazar and Ithamar to serve as priests with their father, Aaron.
Matthew Henry's Commentary on Numbers 3:4
Commentary on Numbers 3:1-13
(Read Numbers 3:1-13)
There was much work belonging to the priests' office, and there were now only Aaron and his two sons to do it; God appoints the Levites to attend them. Those whom God finds work for, he will find help for. The Levites were taken instead of the first-born. When He that made us, saves us, as the first-born of Israel were saved, we are laid under further obligations to serve him faithfully. God's right to us by redemption, confirms the right he has to us by creation.