Jamieson, Fausset and Brown on Jeremiah 23:29

29. As the "fire" consumes the "chaff," [Jr 23:28], so "My word" will consume the false prophets (Mt 3:12; He 4:12). "My word" which is "wheat" [Jr 23:28], that is, food to the true prophet and his hearers, is a consuming "fire," and a crushing "hammer" (Mt 21:44) to false prophets and their followers (2Co 2:16). The Word of the false prophets may be known by its promising men peace in sin. "My word," on the contrary, burns and breaks the hard-hearted (Jr 20:9). The "hammer" symbolizes destructive power (Jr 50:23; Na 2:1, Margin).