No one takes this honor upon himself; he must be called by God, just as Aaron was.

Exodus 28:1
"Next, have your brother Aaron brought to you from among the Israelites, along with his sons Nadab, Abihu, Eleazar, and Ithamar, to serve Me as priests.

Numbers 16:40
just as the LORD commanded him through Moses. This was to be a reminder to the Israelites that no outsider who is not a descendant of Aaron should approach to offer incense before the LORD, lest he become like Korah and his followers.

Numbers 18:7
But only you and your sons shall attend to your priesthood for everything concerning the altar and what is inside the veil, and you are to perform that service. I am giving you the work of the priesthood as a gift, but any outsider who comes near the sanctuary must be put to death."

1 Chronicles 23:13
The sons of Amram: Aaron and Moses. Aaron and his descendants were set apart forever to consecrate the most holy things, to burn incense before the LORD, to minister before Him, and to pronounce blessings in His name forever.

2 Chronicles 26:18
They took their stand against King Uzziah and said, "Uzziah, you have no right to offer incense to the LORD. Only the priests, the descendants of Aaron, are consecrated to burn incense. Leave the sanctuary, for you have acted unfaithfully; you will not receive honor from the LORD God."

John 3:27
John replied, "A man can receive only what is given him from heaven.

And no man takes this honor to himself, but he that is called of God, as was Aaron.