Albert Barnes Commentary Jeremiah 42:5

"Then they said to Jeremiah, Jehovah be a true and faithful witness amongst us, if we do not according to all the word wherewith Jehovah thy God shall send thee to us." — Jeremiah 42:5 (ASV)

Between us - Against us, as in Jeremiah 42:19 (margin).

According to all things - literally, “according to the whole word with which Yahweh your God will send you to us.”