When a prophet speaks in the name of the LORD and the message does not come to pass or come true, that is a message the LORD has not spoken. The prophet has spoken presumptuously. Do not be afraid of him.
If a prophet's word in the LORD's name does not come to pass, the LORD has not spoken it; that prophet has spoken presumptuously and need not be feared.
It answers the question by giving a test for false prophecy and removing its threat.
15The LORD your God will raise up for you a prophet like me from among your brothers. You must listen to him. 16This is what you asked of the LORD your God at Horeb on the day of the assembly, when you said, “Let us not hear the voice of the LORD our God or see this great fire anymore, so that we will not die!” 17Then the LORD said to me, “They have spoken well. 18I 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. 19And I will hold accountable anyone who does not listen to My words that the prophet speaks in My name. 20But if any prophet dares to speak a message in My name that I have not commanded him to speak, or to speak in the name of other gods, that prophet must be put to death.” 21You may ask in your heart, “How can we recognize a message that the LORD has not spoken?” 22When a prophet speaks in the name of the LORD and the message does not come to pass or come true, that is a message the LORD has not spoken. The prophet has spoken presumptuously. Do not be afraid of him.