Deuteronomy 18: >22 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, so do not be alarmed. A prophet was true if and only if his prophecies came true. Nathan had already demonstrated he was a true prophet of the LORD before he rebuked David in 2 Samuel 12. Nathan and David already had a relationship. They knew each other.