Sefer HaChinuch ספר החינוך

That a foreigner not serve in the Temple: That a foreigner not serve in the Temple — meaning to say anyone who is not from the seed of Aharon — as it is stated (Numbers 18:4), “and a foreigner will not approach you.” And the prevention of this is repeated with a different expression in Scripture, and that is as it is stated (Numbers 18:22), “And the Children of Israel shall not again approach the Tent of Meeting, to carry guilt, to die.”

I have written from the roots of the commandment (above) in this Order in the commandment of the service of the Temple (Sefer HaChinukh 394).

From the laws of the commandment is that which they, may their memory be blessed, said that this warning of approaching is only [about] one who approaches to [do] a service from one of the services that are specific to a priest. And it is like they, may their memory be blessed, said (Zevachim 32a), “From the receiving of the blood and onward, it is a commandment of the priesthood.” For example, pourings, mixings, wavings, bringings and many other services like these that are explained in the Gemara that are commanded to be with a priest; behold, they are all with a warning to the foreigner, and the [effected] sacrifice is disqualified. But there are [also] services that are not included in this warning, such as slaughtering, which is fit, even a priori, with a foreigner, and even of sacrifices of the highest sanctity. And so [too,] lighting the lights; such that if a priest took them outside after he arranged them, it is permitted for a foreigner to light them. And there are only four specific services among them that are weighty, such that there is liability for the death penalty for them. And they are throwing, burning incense, placement [of the sacrifice] and pouring libations. And it is about them that it is stated, “and the foreigner that approaches shall die,” as it is elucidated in the first chapter of Yoma [entitled] Kol Mi Shehu Rotseh Litrom (Yoma 24a). [This] and the rest of its details are elucidated there in Yoma and in the last chapter of Tractate Zevachim. (See Mishneh Torah, Laws of Admission into the Sanctuary 3.)

And this prohibition is practiced by males and females at the time of the [Temple]; and even at this time, even though it is desolate, on account of our iniquities. And one who transgresses it and does one of the services that are specific to the priests in the Temple even today, violates a negative commandment, as we have written (Sefer HaChinukh 184, 263). And if he does one of the four services that we mentioned, he is liable for death by the hand of the Heavens.