Albert Barnes Commentary Jeremiah 42:19

"Jehovah hath spoken concerning you, O remnant of Judah, Go ye not into Egypt: know certainly that I have testified unto you this day." — Jeremiah 42:19 (ASV)

The request made in (Jeremiah 42:3) has been fulfilled: Yahweh has spoken. The prophet now adds these four verses as a sort of epilogue, in which he urges upon them the several points of the divine message. In the ten days which had intervened between the request and the answer, Jeremiah had become aware that neither princes nor people were prepared to obey unless the answer was in accordance with their own wishes. Therefore, he does his best to convince them, but as usual, it was his lot to speak the truth to willful men, and gain no hearing.