5 And Jehovah spoke to Moses, saying, 6 Bring the tribe of Levi near, and present them before Aaron the priest, that they may minister unto him; 7 and they shall keep his charge, and the charge of the whole assembly, before the tent of meeting, to do the service of the tabernacle.
Matthew Henry's Commentary on Numbers 3:5-7
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.