Deuteronomy 13:6

The first case. If a prophet, or one who had dreams, should rise up to summon to the worship of other gods, with signs and wonders which came to pass, the Israelites were not to hearken to his words, but to put him to death. The introduction of חלום חלם, "a dreamer of dreams," along with the prophet, answers the two media of divine revelation, the vision and the dream, by which, according to Num 12:6, God made known His will. With regard to the signs and wonders (mopheth, see at Exo 4:21) with which such a prophet might seek to accredit his higher mission, it is taken for granted that they come to pass (בּוא); yet for all that, the Israelites were to give no heed to such a prophet, to walk after other gods. It follows from this, that the person had not been sent by God, but as a false prophet, and that the signs and wonders which he gave were not wonders effected by God, but σημεῖα καὶ τέρατα ψεύδους ("lying sings and wonders," Th2 2:9); i.e., not merely seeming miracles, but miracles wrought in the power of the wicked one, Satan, the possibility and reality of which even Christ attests (Mat 24:24). - The word לאמר, saying, is dependent upon the principal verb of the sentence: "if a prophet rise up...saying, We will go after other gods."

Further provision is made by this branch of the statute against receiving the infection of idolatry from those that are near and dear to us. I. It is the policy of the tempter to send his solicitations by the hand of those whom we love, whom we least suspect of any ill design upon us, and whom we are desirous to please and apt to conform ourselves to. The enticement here is supposed to come from a brother or child that are near by nature, from a wife or friend that are near by choice, and are to us as our own souls, Deu 13:6. Satan tempted Adam by Eve and Christ by Peter. We are therefore concerned to stand upon our guard against a bad proposal when the person that makes it can pretend to an interest in us, that we many never sin against God in compliment to the best friend we have in the world. The temptation is supposed to be private: he will entice thee secretly, implying that idolatry is a work of darkness, which dreads the light and covets to be concealed, and in which the sinner promises himself, and the tempter promises him, secrecy and security. Concerning the false gods proposed to be served, 1. The tempter suggests that the worshipping of these gods was the common practice of the world; and, if they limited their adorations to an invisible Deity, they were singular, and like nobody, for these gods were the gods of the people round about them, and indeed of all the nations of the earth, Deu 13:7. This suggestion draws many away from religion and godliness, that it is an unfashionable thing; and they make their court to the world and the flesh because these are the gods of the people that are round about them. 2. Moses suggests, in opposition to this, that it had not been the practice of their ancestors; they are gods which thou hast not known, thou nor thy fath…

13:6 neither you nor your ancestors have known: The Israelites had heard of these false gods before, but they had never worshiped them. Doing so would violate the first two commandments, the essence of the covenant.