And the word of the LORD came unto me, saying, ‘Son of man, prophesy against the prophets of Israel that prophesy, and say thou unto them that prophesy out of their own hearts, ‘Hear ye the word of the LORD; ‘Thus saith the Lord GOD; ‘Woe unto the foolish prophets, that follow their own spirit, and have seen nothing! O Israel, thy prophets are like the foxes in the deserts. Ye have not gone up into the gaps, neither made up the hedge for the house of Israel to stand in the battle in the day of the LORD. They have seen vanity and lying divination, saying, ‘The LORD saith:’ and the LORD hath not sent them: and they have made others to hope that they would confirm the word. Have ye not seen a vain vision, and have ye not spoken a lying divination, whereas ye say, ‘The LORD saith it;’ albeit I have not spoken? Therefore thus saith the Lord GOD; ‘Because ye have spoken vanity, and seen lies, therefore, behold, I am against you, saith the Lord GOD. And Mine hand shall be upon the prophets that see vanity, and that divine lies: they shall not be in the assembly of My people, neither shall they be written in the writing of the house of Israel, neither shall they enter into the land of Israel; and ye shall know that I am the Lord GOD.'” Ezekiel 13:1-9 (KJV)
Vain preaching is attempting to preach anything except what God says; and claim the “Thus saith the LORD” authority. God says, “Mine hand shall be upon the prophets that see vanity, and that divine lies: they shall not be in the assembly of My people…” Woe to the preacher; of any day; who does not preach the Word of God.