Matthew Henry Commentary Deuteronomy 13:1-5

"If there arise in the midst of thee a prophet, or a dreamer of dreams, and he give thee a sign or a wonder, and the sign or the wonder come to pass, whereof he spake unto thee, saying, Let us go after other gods, which thou hast not known, and let us serve them; thou shalt not hearken unto the words of that prophet, or unto that dreamer of dreams: for Jehovah your God proveth you, to know whether ye love Jehovah your God with all your heart and with all your soul. Ye shall walk after Jehovah your God, and fear him, and keep his commandments, and obey his voice, and ye shall serve him, and cleave unto him. And that prophet, or that dreamer of dreams, shall be put to death, because he hath spoken rebellion against Jehovah your God, who brought you out of the land of Egypt, and redeemed thee out of the house of bondage, to draw thee aside out of the way which Jehovah thy God commanded thee to walk in. So shalt thou put away the evil from the midst of thee." — Deuteronomy 13:1-5 (ASV)

Moses had cautioned against the danger that might arise from the Canaanites. Here he cautions against the rise of idolatry among themselves.

It is necessary for us to be well acquainted with the truths and precepts of the Bible, because we may expect to be tested by temptations of evil disguised as good and by error presented in the guise of truth; indeed, nothing but the plain, express testimony of God's word to the contrary can rightly oppose such temptations. Furthermore, it would be a proof of sincere affection for God if, despite specious pretenses, they were not persuaded to forsake God and follow other gods to serve them.