The 21st mitzvah is that we are commanded to have a tremendous amount of awe for the Holy Temple, to the ex­tent that the fear and trembling becomes set in our hearts. This mitzvah is known as mora'as HaMikdash.

The source of this mitzvah is G‑d's statement1 (exalted be He), "Revere My Sanctuary."

The exact nature of this mitzvah is described in the Sifra:2 "What constitutes awe? Do not enter the Temple Mount carry­ing a walking stick, or wearing shoes, or clad in an undershirt,3 or with dirt on one's feet. Do not use it as a shortcut and cer­tainly do not spit there." It is explained in many Talmudic pas­sages4 that it is absolutely forbidden for one to sit down in the Temple courtyard, with the exception of a king from the House of David. All of these laws are included in this that G‑d (exalted be He) said, "Revere My Sanctuary."

This commandment applies forever, even now when — due to our many sins — it has been destroyed. The Sifra5 says, "From this verse ['Revere My Sanctuary'] I only know that the mitzvah applies when the Sanctuary is standing. What verse tells us that it applies even when there is no Sanctuary? From [the two phrases,] 'Keep My Sabbaths and revere My Sanctuary.' Just as the mitzvah of keeping the Sabbath is forever, so too the mitzvah of having awe for the Sanctuary." They also explain there, "The awe is not for the Sanctuary itself, but for the One who gave the command regarding the Sanctuary."6