therefore

Leviticus 21:21-23
No descendant of Aaron the priest who has a defect shall approach to present the offerings made by fire to the LORD. Since he has a defect, he is not to come near to offer the food of his God. . . .

Numbers 3:10
So you shall appoint Aaron and his sons to carry out the duties of the priesthood; but any layman who approaches the tabernacle must be put to death."

Numbers 16:40
just as the LORD commanded him through Moses. This was to be a reminder for 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 for 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."

Ezekiel 44:10-14
Surely the Levites who wandered away from Me when Israel went astray, and who wandered away from Me after their idols, will bear the consequences of their iniquity. . . .