Therefore hear, ye nations, and know, O congregation, what is among them.
Congregation - parallel to "nations;" it therefore means the gathered peoples who invited to be witnesses how great is the perversity of the Israelites (Jeremiah 6:16-17), and that they deserve the severe punishment about to be inflicted on them (Jeremiah 6:19).
Know what is among them - what deeds are committed by the Israelites (Jeremiah 6:16-17). (Maurer.) Or, 'what punishments are about to be inflicted on them' (Calvin). I prefer the former view.