Albert Barnes Commentary Jeremiah 6:18

"Therefore hear, ye nations, and know, O congregation, what is among them." — Jeremiah 6:18 (ASV)

God summons three witnesses to hear His sentence.

1. The Gentiles.
2. All mankind, Jews and Gentiles.
3. Nature .

What is among them – Rather, “what happens” in them; that is, “Know what great things I will do to them.”