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; (Numbers 3:12)
Anciently the father in the family performed the priestly functions (Ex. 13:8; Judges 17:10), and passed that office on to the first-born son. That custom is now replaced by the appointment of the Levites, who had rallied to Moses at the time of the worship of the golden calf (Ex. 32:26).