18. congregation—parallel to "nations"; it therefore means the gathered peoples who are invited to be witnesses as to how great is the perversity of the Israelites (Jer 6:16, 17), and that they deserve the severe punishment about to be inflicted on them (Jer 6:19).
what is among them—what deeds are committed by the Israelites (Jer 6:16, 17) [Maurer]. Or, "what punishments are about to be inflicted on them" [Calvin].