Numbers 18:7

Therefore thou and thy sons with thee shall keep your priest's
office
Observe all the duties of it, and keep out others from intruding into it: for everything of the altar:
both of incense and of burnt offering with respect to burning incense on the one, and offering sacrifices on the other; both were to be done by priests, and by no other: and within the vail;
in the most holy place, where the high priest entered but once a year, and he only with incense, and the blood of sacrifices, see ( Hebrews 9:7 ) ; and ye shall serve;
do all the business that is to be done at either altar, whether in the court, or in the holy place, and whatsoever is to be done in the most holy place within the vail: I have given your priest's office [unto you] as a service of gift;
it was not what they had taken to themselves of their own will, or had thrust themselves into, but what the Lord had called them to, and had freely invested them with, see ( Hebrews 5:4 ) ; and the stranger that cometh nigh shall be put to death;
any common person, as the Targums of Onkelos and Jonathan; any Israelite, one that is a stranger from the priests, though a Levite, as Aben Ezra; such an one might not come either to the altar of burnt offering to offer any sacrifice upon it, or the altar of incense, to burn incense on that, or trim the lamps, or put the shewbread in order, or to do anything peculiar to the priest's office.