What if anyone other than a descendent of Aaron did priestly duties? The Old Testament gave a firm answer. The Levites were appointed to be helpers for their relatives, the Aaronic priests. But even the Levites could not touch the holiest things. If they or anyone else acted as priests, they deserved the death penalty.
“Any outsider who comes near shall be put to death.”
You shall appoint Aaron and his sons, and they shall guard their priesthood. But if any outsider comes near, he shall be put to death. (Numbers 3:10)
[The LORD said to Aaron,] “I have taken your brothers the Levites from among the people of Israel. They are a gift to you, given to the LORD, to do the service of the tent of meeting. And you and your sons with you shall guard your priesthood for all that concerns the altar and that is within the veil; and you shall serve. I give your priesthood as a gift, and any outsider who comes near shall be put to death.” (Numbers 18:6-7)
Korah was a Levite who failed to see the difference between Aaron and himself. He tried to “seek the priesthood too” (Numbers 16:10). God Himself destroyed Korah and his followers (Number 16:1-35). Much later, king Uzziah of Judah tried to take a priestly role, and was punished with slow death by leprosy. The true priests confronted him and said,
It is not for you, Uzziah, to burn incense to the LORD, but for the priests, the sons of Aaron, who are consecrated to burn incense. Go out of the sanctuary, for you have done wrong, and it will bring you no honor from the LORD God. (2 Chronicles 26:18)