Throughout Genesis, it was the tribal patriarch who functioned as the priest for the family. The exception to this was Melchizedek, who was identified as “priest of God Most High” (Gen. 14:18). In Exodus, we meet Moses’ father-in-law, who is introduced as “the priest of Midian” (Ex. 2:16). Once the people of Israel arrived at Sinai, however, the situation changed significantly.
In Exodus 28:1, God told Moses to set apart Aaron and his sons from among all the people of Israel to serve the Lord as priests. The priestly garments that were to be made were made for the use of Aaron and his sons in their role as priests. Exodus 29 gives the directions for the consecration of Aaron and his sons as priests. This identification of Aaron and his sons as priests for Israel is consistent through the remaining chapters of Exodus and is summed up in Exodus 40:9–15, which identifies Aaron and his sons as admitted to a perpetual priesthood throughout their generations. Throughout the book of Leviticus, the priests are regularly identified as the sons of Aaron (see, for example, Leviticus 1:5, 8; 21:1). This identification of the priesthood with Aaron and his sons occurs also in Numbers (Num. 3:3, 10; 18:1; 26:1).
The book of Deuteronomy appears to complicate the issue by using the term “Levitical priests” (Deut. 17:9, 18; 24:8; 27:9). This seems to imply that all the Levites were priests. The wording of Deuteronomy 18:1 especially gives this impression: “The Levitical priests, all the tribe of Levi, shall have no portion or inheritance with Israel.” The language appears to identify the entire tribe of Levi with the priesthood. Some conclude on this basis that there was a priesthood of Aaron and a Levitical priesthood—two different priesthoods.
But such a view fails to take some things into account. First, Aaron (and Moses) was of the tribe of Levi (Ex. 6:14–27). Therefore, he is technically a priest of the tribe of Levi. Second, the Levites were set aside as servants to the priests, representing the entire body of the people of Israel (Num. 8:5–22). The Levites replaced and represented the firstborn of Israel (Ex. 13:2; Num. 3:11–13). In that sense, the entire tribe of Levi served a priestly function, but only Aaron and his descendants had the priestly office. Third, the regulations regarding the priests in Exodus, Leviticus, and Numbers were provided primarily for the instruction of the priests. It would thus be important to make it clear that these rules applied to those who held the office of the priest. The regulations in Deuteronomy are directed at the whole population of Israel. The difference between Levite and priest is minimized in order to elevate the role of the whole priestly tribe in the eyes of the people.
Some people point to one possible exception to this analysis. The prophet Samuel also served as a priest in the tabernacle, yet his family was from the hill country of Ephraim, from the town of Ramathaim-Zophim. This is not listed as one of the towns assigned to the Levites in Joshua 21. In addition, the Levites who were assigned cities in Ephraim were not of the line of Aaron. Did Samuel legitimately serve as a priest? First Chronicles 6:16–48 includes Samuel as a descendant of Kohath, from whom Aaron and Moses also descended. The Scripture never explicitly identifies Samuel as a priest, but Psalm 99:6 includes him with Moses and Aaron, who “were among his priests.” In addition, it seems likely that families moved around in the turbulent days of the judges, as Elimelech moved his family to Moab during the famine (Ruth 1:1–5). Though it cannot be shown definitively that Samuel was a descendant of Aaron, the information Scripture gives us makes it entirely likely.
After the time of David, the priestly family was limited to the line of Zadok. This served to fulfill the judgment that God pronounced on Eli for his failure regarding his sons. (See 1 Samuel 2:27–36 for the judgment pronounced. See 1 Kings 2:26–27, 35 for the specific fulfillment of the judgment.) We see this further confirmed in Ezekiel’s new temple visions, where the Levitical priests are specified to be the sons of Zadok (Ezek. 44:15).
As a result, when we come to the issue of Levitical and Aaronic priesthoods, we find that we are using two different names for the same thing. The priestly office in ancient Israel belonged to the descendants of Aaron, though eventually it was limited to the line of Zadok. This family was part of the tribe of Levi through Kohath, one of the sons of Levi (Num. 3). The remaining families of the descendants of Levi provided men for the role of Levite, whose duties were set out in Numbers 4. Primarily they served as assistants to the priests. Those duties were changed by David as he made preparation for the building of the temple (1 Chron. 23–24). In short, the tribe of Levi took the place of the firstborn of Israel. In that way, the entire tribe filled a priestly role, but only the descendants of Aaron held the office of priest.