John Gill Commentary Jeremiah 29:9

"For they prophesy falsely unto you in my name: I have not sent them, saith Jehovah." — Jeremiah 29:9 (ASV)

For they prophesy falsely to you in my name They pretended to have the authority of God for what they said; that their prophecies and dreams were from him, and as such they delivered them in his name; though they were false ones; that they might be the better received by the people:

I have not sent them, says the Lord ; they had no mission or commission from the Lord, no warrant or authority from him; they set up themselves; and ran without being sent; and prophesied out of their own hearts what came into their heads, the fancies of their own brain, or the delusions of Satan, under whose power and influence they were;

therefore sad must be the case of a people giving heed to such seducing spirits.