Ezra 2:62
These searched [for their names] among those registered in the genealogies, but they were not found; so they were excluded from the priesthood as [ceremonially] unclean.
Numbers 3:10
So you shall appoint Aaron and his sons, and they shall observe and attend to their priesthood; but the layman (non-Levite) who approaches [the holy things] shall be put to death.”
Leviticus 21:21-23
No man among the descendants of Aaron the priest who has a [physical] defect and is disfigured or deformed is to approach [the altar] to present the offerings of the Lord by fire. He has a defect; he shall not approach [the altar] to present the food of his God.
Numbers 16:39-40
So Eleazar the priest took the bronze censers which the Levites who were burned had offered, and they were hammered out [into broad sheets] as a plating for the [bronze] altar [of burnt offering],
Numbers 18:7
Therefore you and your sons with you shall attend to your priesthood for everything concerning the altar [of burnt offering and the altar of incense] and [of the Holy of Holies] within the veil, and you are to serve. I am giving you the priesthood as a bestowed service, but the stranger (outsider, layman) who approaches shall be put to death.”
Ezekiel 44:10-14
But the Levites who went far away from Me when Israel went astray, who went astray from Me after their idols, they shall bear the punishment for their sin and guilt.