Here are 2 Bible verses about false witness in preaching, curated from both the Old and New Testaments of the Holy Bible, King James Version, arranged from most to least relevant.
John 4:23 KJV
23 But the hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship him.
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Matthew 24:24 KJV
24 For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.