Charles Ellicott Commentary Jeremiah 9:3

"And they bend their tongue, [as it were] their bow, for falsehood; and they are grown strong in the land, but not for truth: for they proceed from evil to evil, and they know not me, saith Jehovah." — Jeremiah 9:3 (ASV)

Like their bow for lies. —The inserted words turn the boldness of the metaphor into a comparatively tame simile. They bend their tongue to be their bow of lies. The same figure is found in Psalm 57:4; Psalms 58:7; Psalms 64:3.

They are not valiant for the truth upon the earth. —Better, they are not mighty for truth, that is, faithfulness, in the land — that is, they do not rule faithfully. It is not without some regret that we set aside a phrase which has gained something of a proverbial character as applied to the champions of speculative truth or abstract right, but the above gives the true meaning of the Hebrew.

They know not me. —“Know” in the sense of acknowledging and obeying (1 Samuel 2:12; Job 18:21). This was the root evil from which all other evils arose.