From BiblePortal Wikipedia

Priests [1]

Functions Priesthood in the Old Testament primarily involved sacrificing at the altar and worship in the shrine. Other functions were blessing the people ( Numbers 6:22-26 ), determining the will of God ( Exodus 28:30 ), and instructing the people in the law of God ( Deuteronomy 31:9-12 ). This instruction included the application of the laws of cleanness ( Leviticus 11-15 ). Some of these functions, like blessing and teaching, would not be reserved for priests alone, but sacrificing and the use of the Urim and Thummim were theirs exclusively. See  Leviticus 11-15 .

If the main characteristic of priesthood was sacrificing, the office is as old as Abel. Noah sacrificed; so did Abraham and the patriarchs. We may say that they were family priests. Jethro, the priest of Midian, brought sacrifices to God and worshiped with Moses, Aaron, and the elders of Israel ( Exodus 18:12 ). God promised that Israel, if it were faithful, would be a “kingdom of priests, a holy nation” ( Exodus 19:6 ). This may have meant that Israel was called to mediate God's word and work to the world—to be a light to the nations ( Isaiah 42:6 ).

Later, when God purposed to establish the nation, He chose Moses to organize the army, to set up a system of judges, to build a house of worship, and to ordain priests to serve therein. The formal priesthood goes with the formal worship of an organized nation of considerable size. On Mount Sinai, God gave Moses instructions to build the tabernacle. On the mount, God told Moses to appoint Aaron and his four sons to serve as priests, that is, to serve at the altar and in the sanctuary ( Exodus 28:1 ,Exodus 28:1, 28:41 ). Their holy garments are prescribed in detail and their consecration ritual is given in  Exodus 28:1 and   Exodus 29:1 . As to the work of these priests, most of Leviticus and some of Numbers and Deuteronomy give details. Aaron and his descendants of the tribe of Levi served in the tabernacle and Temple as priests. Members of the tribe of Levi not related to Aaron assisted the priests but did not offer sacrifices. Priests were supported by offerings and Levites by tithes ( Numbers 18:20-24 ). See Levites; High Priest; Aaron .

R. Laird Harris