The story
From a phrase in Revelation 2:9 (also Revelation 3:9), where the text describes "the blasphemy of those who say they are Jews, and are not, but are the synagogue of Satan." (Revelation 2:9). The expression was subsequently used polemically for various religious groups before also acquiring a modern antisemitic sense referring to Jews collectively.
Etymology from Wiktionary, CC BY-SA 4.0.