“The most general concept of priesthood is centered in the notion of sacrifice. According to this concept, anyone who puts aside from his own use something he wishes to consecrate to the divinity is considered a priest. This is the original meaning of the expression sacrum facere.
Holy Scripture provides several instances of natural priesthood, one of which is the case of Abel, the ‘just man,’ whose sacrifice is commemorated daily in the Mass (Gn. 4:4). Natural priesthood is most often exercised by the leaders of a people in an act of public worship to obtain blessing for them. This office naturally falls, then, to the head of a family and to the qualified representatives of an ethnic group or clan.
Later, the sacrificial offering became the privilege of kings. Melchisedec, the high priest, was king of Salem (Gn. 14:18). The Babylonian and Assyrian leaders took pride in their priestly dignity; among the titles of honor attributed to them, that of priest was of exceptional importance. In Egypt, Pharaoh called himself ‘priest of all the gods,’ the qualified mediator between the divinities and his people. It is also true that, with the exception of a few cases, these priest-kings chose certain curates to replace them in the sacerdotal functions. In the organization of the tribes, as we read in Sacred Scripture, each family formed a small ceremonial community whose head was usually its priest. Thus, we see Abraham (Gn. 12:8, 15:8-17), Isaac, after the death of his father (Gn. 26:25), Jacob (Gn. 33:20), and Job (Jb. 1:5) offering sacrifices to the Almighty. It is to be expected that the Israelites would keep this usage in Egypt; it is also likely that the example of a privileged priestly caste, such as existed there, would influence them. This latter probability is enhanced by the fact that Joseph allied himself to the priestly nobility of the country of the Pharaohs by his marriage to Aseneth, daughter of Putiphare, priest of On (Heliopolis) (Gn. 41: 45).
Even under the Mosaic Law the ancient custom of authorizing the heads of families and of clans to fulfill the office of priest or of choosing replacements persisted. We see this in the example of Micha (Jgs. 7:5) who conferred the priestly investiture on one of his sons. But the sequel to that narration shows that this privilege of the Levites was recognized, since Micha subsequently invested an itinerant Levite and accounted himself of more worth in the eyes of God for having done so: ‘Now I know that the Lord will prosper me, since the Levite has become my priest’ (Jgs. 17:13).
The privilege of the royal priesthood was maintained in Israel for a long time. David exercised it in transferring the holy ark to Sion. He donned the priestly vestment (2 Sm. 16:14; 16:20); he offered sacrifices (2 Sm. 6:13, 17) and blessed the people (2 Sm. 6:18)—all functions reserved to the priests (Nm. 6:22-27; Lv. 9:22-23; 3:10; 18:7). His sons were priests (2 Sm. 8:18), that is, substitutes for their father in the sacerdotal offices. Solomon, in his turn, filled the ministry with authority (3 Kgs. 3:4, 14; 8:14-15; 30:55). Besides this, he appointed and removed the priests, considering them his functionaries (3 Kgs. 2:26-27; 35). The custom continued under Jeroboam I (3 Kgs. 12:33) and Achaz (4 Kgs. 5:12ff.). The sacerdotal reform which took place under Josias (640-609 B.C.) put an end to this privilege of the kings. At that time the cult was centralized at Jerusalem (Dt. 12:1-14; 4 Kgs. 23), an innovation indeed, which was not yet in effect under the Judges (Jgs. 6:28, 13:16 ff.), nor even under Solomon (3 Kgs. 3:4).”
-Clement Dillenschneider, C.SS.R. in Christ the One Priest and We His Priests.