Before Aaron was chosen for the priesthood, every member of Israel was eligible to serve as a priest. The entire nation stood on equal footing when it came to approaching God through sacrificial service. But once Aaron was singled out, everyone else was excluded.
The Mekhilta supports this with two decisive verses. First, (Numbers 18:19): "An everlasting covenant of salt is it before the Lord for you and for your sons." The "covenant of salt" — an unbreakable, permanent agreement — was made exclusively with Aaron and his descendants. Second, (Numbers 25:13): "And it shall be unto him and to his seed after him the covenant of an everlasting priesthood." Both verses use the word "everlasting," emphasizing that this selection was permanent and irreversible.
This teaching fits into a broader Mekhilta pattern about divine choosing. Just as God narrowed the field when selecting the land of Israel from all lands, and Jerusalem from all cities within that land, He narrowed the priesthood from all of Israel down to one family. The rabbis saw this as a fundamental principle of how holiness operates in the world. God does not leave sacred roles open to everyone indefinitely. He selects, and that selection creates permanent categories — those who serve at the altar and those who do not. The choosing of Aaron was not arbitrary favoritism but the establishment of a divinely ordained order that would structure Jewish worship for generations.