Charles Ellicott Commentary Jeremiah 23:29

"Is not my word like fire? saith Jehovah; and like a hammer that breaketh the rock in pieces?" — Jeremiah 23:29 (ASV)

Is not my word like as a fire? ... —The prophet speaks out of the depths of his own experience. The true prophetic word burns in a person's heart and will not be restrained (Jeremiah 5:14; Jeremiah 20:9; Psalms 39:3), and when uttered, it consumes the evil and purifies the good. It will also burn up the chaff of the false prophets' utterances (Compare to 1 Corinthians 3:12–13).

As the hammer breaks the rock, so the word shatters human pride and stubbornness. It is mighty to the pulling down of strongholds (2 Corinthians 10:4), and the heart of the one who hears it as it should be heard is broken and contrite.

What these words paint in the language of poetry, Saint Paul describes without imagery in 1 Corinthians 14:24–25 (Compare also to Hebrews 4:12).