“Jewish” is an ethnicity in addition to a belief system.
The antisemites like to point that out when they discuss Marxism, since Marx, although he was atheist and his father Heinrich (born Herschel Levy) was a convert to Lutheranism, had a paternal grandfather (Marx Levy Mordechai) who was rabbi of Trier. So all “Bolshevism” they falsely characterize as Jewish, assigning guilt by association via their racist outlook.