4 And Nadab and Abihu died before Jehovah, when they offered strange fire before Jehovah, in the wilderness of Sinai, and they had no children; and Eleazar and Ithamar ministered in the priest's office in the presence of Aaron their father.

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.