Evidently the family heads acted as priests among the offspring of Jacob (Israel) until the Levitical priesthood was established by God. Hence, when God led the people to Mount Sinai he commanded: “Let the priests also who regularly come near to Jehovah sanctify themselves, that Jehovah may not break out upon them.” (Ex 19:22) This was before the Levitical priesthood was established. But Aaron, though not yet designated as priest, was allowed to go partially up the mountain with Moses. This circumstance harmonized with the later appointment of Aaron and his posterity as priests. (Ex 19:24) Seen in retrospect, this was an early indication that God had in mind a superseding of the old arrangement (of family-head priesthood) by means of a priesthood of Aaron’s house.

Prior to this transaction Jehovah had already set apart the male members of the family of Aaron of the tribe of Levi as constituting the priesthood of Israel.​—Nu 1:1; 3:6-10.