363. And Stay Out! The prohibition against Impure people entering the Temple
…they shall not contaminate… (Numbers 5:3)
The previous mitzvah was a positive command for those affected by various forms of ritual impurity to remove themselves from the machaneh Shechinah, the camp containing the Mishkan (Tabernacle) or, later, the Temple and its courtyard. Here, we have a negative command prohibiting these people from entering such areas.
The underlying rationale for this prohibition is what we said in the previous mitzvah: ritual impurity works at cross-purposes with Temple sanctity. The Temple service requires a high degree of purity, so we are not to contaminate such efforts.
Even today, one may not ascend the Temple mount in a state of ritual impurity.
This mitzvah is discussed in the Talmud in the tractates of Pesachim (66b-68a), Makkos (14b-15a), and elsewhere. It is codified in the Mishneh Torah in the third chapter of Hilchos Biyas HaMikdash and is #77 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos.