Wherefore thus saith the LORD God of hosts, Because ye speak this word, behold, I will make my words in thy mouth fire, and this people wood, and it shall devour them.
Because ye speak this word. He turns away from addressing the people directly, to the prophet: implying that He puts them to a distance from Him and only communicates with them through His prophet (Jeremiah 5:19): He began with saying "Ye," but ends with speaking of them, no longer to them: "this people!" estranged from me.
Behold, I will make my words in thy mouth fire, and this people wood - thy denunciations of judgments shall be fulfilled, and shall consume them as fire does wood. In Jeremiah 23:29 it is the penetrating energy of fire which is the point of comparison: here it is its devouring violence.