Numbers 3:1-13
1 These also are the generations of Aaron and Moses in the day that the LORD spake with Moses in mount Sinai.
2 And these are the names of the sons of Aaron; Nadab the firstborn, and Abihu, Eleazar, and Ithamar.
3 These are the names of the sons of Aaron, the priests which were anointed, whom he consecrated to minister in the priest's office.
4 And Nadab and Abihu died before the LORD, when they offered strange fire before the LORD, in the wilderness of Sinai, and they had no children: and Eleazar and Ithamar ministered in the priest's office in the sight of Aaron their father.
5 And the LORD spake unto 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 congregation before the tabernacle of the congregation, to do the service of the tabernacle.
8 And they shall keep all the instruments of the tabernacle of the congregation, and the charge of the children of Israel, to do the service of the tabernacle.
9 And thou shalt give the Levites unto Aaron and to his sons: they are wholly given unto him out of the children of Israel.
10 And thou shalt appoint Aaron and his sons, and they shall wait on their priest's office: and the stranger that cometh nigh shall be put to death.
11 And the LORD spake unto Moses, saying,
12 And I, behold, I have taken the Levites from among the children of Israel instead of all the firstborn that openeth the matrix among the children of Israel: therefore the Levites shall be mine;
13 Because all the firstborn are mine; for on the day that I smote all the firstborn in the land of Egypt I hallowed unto me all the firstborn in Israel, both man and beast: mine shall they be: I am the LORD.
44-51, the Substitute for the First-born
The Levites were taken, instead of the firstborn sons of Israel, to perform the service of the Tabernacle. There were nearly as many Levites as first-born, and the difference was made good by the payment of redemption money on the part of the overplus, Numbers 3:45. So each of us has been redeemed by the blood of the Lamb to belong to God and to minister to Him! Aaron's family provided the priesthood, Leviticus 3:9. The rest of the tribe performed the more irksome duties of ministry and transport.
The Gershonites, as eldest, pitched westward of the Tabernacle, and cared for the hangings and curtains. The Kohathites, because related to Aaron, pitched southward, and had charge of the sacred vessels. The Merarites pitched northward, and had charge of the boards and sockets. It was enough for each to know that his place and charge were God-appointed. Even to carry a pin became honorable, when God had allotted that to a man as his share.