Or HaChaim on Deuteronomy 13:2/אור החיים על דברים י״ג:ב׳

1 כי יקום בקרבך נביא, "When a prophet arises in your midst, etc." The word נביא implies that even if this person has an established reputation as a true prophet, if he asks you to violate any of G'd's commandments even temporarily and it involves idolatry he is guilty of death. [He may ask you to temporarily violate a different one of the commandments such as Elijah at Mount Carmel who rebuilt a private altar upon which it was forbidden to offer sacrifices after the Temple in Jerusalem had been built. Ed.]

2ונתן אליך אות או מופת, "and he will demonstrate to you a sign or a wonder, etc." Why did the Torah not first mention what it is the prophet said and which he wishes to lend credence to by performing a miracle? Furthermore, what does the Torah mean when writing: ובא האות…לאמור, and the sign comes about….saying?" Does the sign then say something? Is it not the prophet who says something, i.e. to serve an alien deity, and that is why he incurs the death penalty?

3It appears that our verse addresses the following situation. The prophet claims that G'd has instructed him to legitimise himself by means of a sign or a certain kind of miracle. The idea is that the prophet tells the people that if, as a result of his attempts to create a miracle and this miracle would materialise in a certain predetermined manner, then this would be proof that G'd had asked the people to worship a certain deity. If, on the other hand, said miracle would materialise in a different form, this would be proof that G'd wanted said deity to be burned and destroyed. ובא האות והמופת, and now the miracle has indeed materialised in the form which would indicate that the people ought to worship the deity which the prophet spoke about (although he had been careful to leave the decision to the people themselves), the Torah says: "do not hearken to him!" In other words, such a so-called prophet is guilty of the death penalty even though he himself had not insisted that the people engage in idolatry but had left the decision up to them and to their interpretation of the miracle's meaning. Although the people themselves had freely decided to engage in worship of such a deity, the "prophet" is still guilty of the death penalty.