The 74th prohibition is that a zar is forbidden from serving [in the Temple]. The term zar refers to anyone who is not a descendant of Aharon [i.e., a non-Kohen].
The source of this prohibition is G‑d's statement1 (exalted be He), "A zar may not come close to you [to perform the Temple service]."
Scripture states explicitly that one who transgresses this prohibition is punished by a heavenly death penalty in the verse,2 "A zar who performs the service shall die." The Sifri says, "The verse 'A zar who performs the service shall die,' refers to one who performs the Temple service. This teaches the punishment, but what is the actual prohibition? That is the verse, 'A zar may not come close to you.' " Both the prohibition and the punishment are repeated in the verse,3 "The Israelites shall therefore no longer come forth to the ohel moed, since they can then become guilty of sin and die."
In tractate Yoma,4 it is explained for which types of service a zar is punished by death: "A zar is punishable by death for the following four types of service — sprinkling [the blood on the altar], burning [the fats, etc. on the altar], spilling wine [on the altar], and spilling water [on the altar]."
The details of this mitzvah are explained there and in the last chapter of tractate Zevachim.5
