The truth is that being Jewish is both a religious and ethnic classification
depending on context. Nationality is not really a good description as the Jews
have since the First Exile in Babylon always had a Diaspora. The peculiar Jewish
situation stems >from the time when every people had its own religion. There were
even more local deities too. When someone moved to a new place he adopted the
faith of the place. So when NonJews moved to Ancient Israel they became Jewish.
The difference was Judaism rejected other gods and so could survive in Exile. We
started the concept of absolute religion/ethics as opposed to pleasing local
deities with differing demands. If it were as simple as Religion or Ethnicity
we would not be having anything to discuss. No one calls the at least global
Catholic community an ethnic group.