 Aaron and his four sons were the first priests appointed as the priestly system was established by God.The Levites as a tribe were later ordained for the priestly service after answering a call to take the ’s side after the idolatry centered around the golden calf.After the death of Nadab and Abihu, Eleazar and Ithamar took their places as priests, because neither Nadab nor Abihu had any sons.