Deuteronomy 13:1-3 - If a prophet, or one who foretells by dreams, appears among you and announces to you a miraculous sign or wonder, and if the sign or wonder of which he has spoken takes place, and he says, "Let us follow other gods" - "and let us worship them," you must not listen to the words of that prophet or dreamer. The LORD your God is testing you to find out whether you love him with all your heart and with all your soul.
13:5 That prophet or dreamer must be put to death, because he preached rebellion against the LORD your God, who brought you out of Egypt and redeemed you from the land of slavery; he has tried to turn you from the way the LORD your God commanded you to follow. You must purge the evil from among you.
13:6, 8-11 If your very own brother, or your son or daughter, or the wife you love, or your closest friend secretly entices you, saying, "Let us go and worship other gods" do not yield to him or listen to him. Show him no pity. Do not spare him or shield him. You must certainly put him to death. Your hand must be the first in putting him to death, and then the hands of all the people. Stone him to death, because he tried to turn you away from the LORD your God, who brought you out of Egypt, out of the land of slavery. 11 Then all Israel will hear and be afraid, and no one among you will do such an evil thing again.
Seriously, just read all of chapter 13!!
Deuteronomy 18:17-22) The LORD said to me: "What they say is good. I will raise up for them a prophet like you from among their brothers; I will put my words in his mouth, and he will tell them everything I command him. If anyone does not listen to my words that the prophet speaks in my name, I myself will call him to account. But a prophet who presumes to speak in my name anything I have not commanded him to say, or a prophet who speaks in the name of other gods, must be put to death."
You may say to yourselves, "How can we know when a message has not been spoken by the LORD ?" If what a prophet proclaims in the name of the LORD does not take place or come true, that is a message the LORD has not spoken. That prophet has spoken presumptuously. Do not be afraid of him.
Hebrews 1:1-3) In the past God spoke to our forefathers through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom he made the universe. The Son is the radiance of God's glory and the exact representation of his being, sustaining all things by his powerful word.-
Last one, because there's too many!
Matthew 7:15 Watch out for false prophets. They come to you in sheep's clothing, but inwardly they are ferocious wolves.
