In the Bible’s earliest chapters the heads of the families seem to function as priests on behalf of their relatives (Genesis 6:9-22; 8:20; Job 1:1-5). However, the Bible first introduces the term “priest” in Genesis 14. Abraham was returning as a victor from war. He was met by two very different kings. Abraham wanted little to do with the king of the wicked city Sodom. The other king was Melchizedek of Salem, the epitome of good. “He was priest of God Most High” (Genesis 14:18). In this role, Melchizedek blessed Abraham. Melchizedek was not a Jew, for Israel and his nation had not yet been born. Much later, the New Testament reveals that Melchizedek was God’s way of preparing people to understand the priesthood of Jesus (Hebrews 5-7).
Idol worshipers appointed their own priests (Genesis 41:45; 47:22; 2 Kings 23:5; Zephaniah 1:4 cf. Judges 17:1-13). Yet, even among Gentiles, the true God still had followers and priests, of whom Jethro, also called Reuel, was an example. He was “priest of Midian” (Exodus 2:16; 3:1; 18:1). If there were other priests like Jethro, the Bible says little or nothing about them.
By contrast, the Old Covenant went into great detail about the priests for Israel. These priests, as a group distinct from ordinary Israelites, were first anticipated in Exodus 19:22-24. Soon after that, Exodus 28 showed how to appoint priests. By God’s order, the priesthood belonged exclusively to the family of Aaron, a small group within the Jewish tribe of Levi.
Bring near to you Aaron your brother, and his sons with him, from among the people of Israel, to serve me as priests—Aaron and Aaron’s sons, Nadab and Abihu, Eleazar and Ithamar. (Exodus 28:1)
You shall gird Aaron and his sons with sashes and bind caps on them. And the priesthood shall be theirs by a statute forever. Thus you shall ordain Aaron and his sons. (Exodus 29:9)
Their anointing shall admit them to a perpetual priesthood throughout their generations. (Exodus 40:15)