Numbers 18:4 "And a stranger shall not come near to you"

Have you ever seen a sign on a door stating: "Authorized Personnel Only"?

This sign, and others like it, warn those who do not have permission to enter a certain area to keep away.

This Negative Mitzvah serves as a "prevention sign" for all unauthorized persons in the Beit HaMikdash.

All those who are not priests are forbidden to carry out any service in the Beit HaMikdash.