“And a stranger may not draw near to you!” (Bamidbar 18:4)
It is a negative commandment that a stranger, referring to anyone who is not descended from Aharon, may not serve in the temple, as it says “And a stranger may not draw near to you!”, and this prohibition is repeated in different words when the verse says “And the children of Yisrael will no longer draw near to the tent of meeting to commit a sin and die!”.
Amongst the roots of this commandment are that it is part of the honor of the King and the place that there should be specified people from a specified tribe to serve Him, and that another person should not enter between them for the service. This is similar to the way earthly kings act, that they appoint for themselves respected known men to perform all the work in the palace, for it is not fitting that a king’s workers should change every day.
Amongst the laws of this commandment are that there is no prohibition unless the stranger actually performs the service of the offerings which are designated only for the priest, which is from the receiving of the blood onwards. Regarding a meal-offering, this refers to the pouring, mixing, waving, and drawing close to the altar as all of these are the obligation of the priest. A stranger who performs them has transgressed a negative commandment and the offering is invalid. However, there are services which may be performed by anyone. These services include slaughtering, bringing firewood for the altar, relighting the used lights from the Menorah outside. There are also four services which if performed by a stranger deserves death. These are sprinkling the blood, burning the intestines, offering on the altar and pouring the libations. About these it says “And the stranger who draws near shall be put to death!”
This prohibition applies to men and to women when the temple was standing, and even nowadays even though the temple has been destroyed because of our sins. One who performs one of these services has transgressed this prohibition, and if it is one of the four special services, he receives death at the hands of heaven.