From Exodus 22:
You shall not permit a female sorcerer to live.
Whoever sacrifices to any god, other than the LORD alone, shall be devoted to destruction.

From Numbers 25:
2 These invited the people to the sacrifices of their gods, and the people ate and bowed down to their gods. 3 Thus Israel yoked itself to the Baal of Peor, and the LORD’s anger was kindled agai nst Israel. 4 The LORD said to Moses, “Take all the chiefs of the people, and impale them in the sun before the LORD, in order that the fierce anger of the LORD may turn away from Israel.” 5 And Moses said to the judges of Israel, “Each of you shall kill any of your people who have yoked themselves to the Baal of Peor.” 6 Just then one of the Israelites came and brought a Midianite woman into his family, in the sight of Moses and in the sight of the whole congregation of the Israelites, while they were weeping at the entrance of the tent of meeting. 7 When Phinehas son of Eleazar, son of Aaron the priest, saw it, he got up and left the congregation. Taking a spear in his hand, 8 he went after the Israelite man into the tent, and pierced the two of them, the Israelite and the woman, through the belly. 

From Deuteronomy 13:
If prophets or those who divine by dreams appear among you and promise you omens or portents, 2 and the omens or the portents declared by them take place, and they say, “Let us follow other gods” (whom you have not known) “and let us serve them,” 3 you must not heed the words of those prophets or those who divine by dreams; for the LORD your God is testing you, to know whether you indeed love the LORD your God with all your heart and soul. 4 The LORD your God you shall follow, him alone you shall fear, his commandments you shall keep, his voice you shall obey, him you shall serve, and to him you shall hold fast. 5 But those prophets or those who divine by dreams shall be put to death for having spoken treason against the LORD your God—who brought you out of the land of Egypt and redeemed you from the house of slavery—to turn you from the way in which the LORD your God commanded you to walk. So you shall purge the evil from your midst. 6 If anyone secretly entices you—even if it is your brother, your father’s son orb your mother’s son, or your own son or daughter, or the wife you embrace, or your most intimate friend—saying, “Let us go worship other gods,” whom neither you nor your ancestors have known, 7 any of the gods of the peoples that are around you, whether near you or far away from you, from one end of the earth to the other, 8 you must not yield to or heed any such persons. Show them no pity or compassion and do not shield them. 9 But you shall surely kill them; your own hand shall be first against them to execute them, and afterwards the hand of all the people. 10 Stone them to death for trying to turn you away from the LORD your God, who brought you out of the land of Egypt, out of the house of slavery. 11 Then all Israel shall hear and be afraid, and never again do any such wickedness. 12 If you hear it said about one of the towns that the LORD your God is giving you to live in, 13 that scoundrels from among you have gone out and led the inhabitants of the town astray, saying, “Let us go and worship other gods,” whom you have not known, 14 then you shall inquire and make a thorough investigation. If the charge is established that such an abhorrent thing has been done among you, 15 you shall put the inhabitants of that town to the sword, utterly destroying it and everything in it—even putting its livestock to the sword

From Deuteronomy 17:
2 If there is found among you, in one of your towns that the LORD your God is giving you, a man or woman who does what is evil in the sight of the LORD your God, and transgresses his covenant 3 by going to serve other gods and worshiping them—whether the sun or the moon or any of the host of heaven, which I have forbidden— 4 and if it is reported to you or you hear of it, and you make a thorough inquiry, and the charge is proved true that such an abhorrent thing has occurred in Israel, 5 then you shall bring out to your gates that man or that woman who has committed this crime and you shall stone the man or woman to death. 20 But any prophet who speaks in the name of other gods, or who presumes to speak in my name a word that I have not commanded the prophet to speak—that prophet shall die.” 