In antiquity, religion was one defining factor of ethnicity, along with language, regional customs, national costume, etc. As Xenophanes famously comments:
With the rise of Christianity, Islam and Buddhism, ethnic religions came to be marginalized as "leftover" traditions in rural areas, referred to as paganism or shirk (idolatry).
The notion of gentiles ("nations") in Judaism reflect this state of affairs, the implicit assumption that each nation will have its own religion. Historical examples include Germanic polytheism, Celtic polytheism, Slavic polytheism and pre-Hellenistic Greek religion.
Contemporary ethnic religions are Shinto of the Japanese people, Judaism of the Jewish people (see: Who is a Jew?), and Hinduism (except for some, comparatively small Hindu movements: see Hindutva: Who is a Hindu?).
Over time, even revealed religion will assume local traits and in a sense will revert to an ethnic religion. This has notably happened in the course of the History of Christianity, which saw the emergence of national churches with "ethnic flavours" such as Germanic, Ethiopian, Armenian, Greek, Russian and others.'''