Albert Barnes Commentary Jeremiah 23:13

"And I have seen folly in the prophets of Samaria; they prophesied by Baal, and caused my people Israel to err." — Jeremiah 23:13 (ASV)

And I have seen folly ... - Rather, “Also I have seen.” The prophet contrasts the prophets of Samaria with those of Jerusalem. In the conduct of the former, God saw folly (literally that which is insipid, as being unsalted). It was stupidity to prophesy by Baal, an idol.

In Baal - that is, in the name of Baal.