I’m not a Christian nor a professional historian, but I have this theory …
Christianity was something new in the ancient Mediterranean –– the wellspring of Western civilization –– because it wasn’t the religion of a people.
Judaism belonged to the descendants and followers of Abraham; it got its name because people in Judea (modern Palestine) practiced it. The Romans had their gods, the Greeks had theirs, the Egyptians had theirs, and so on. Religions were essentially tribal or national, even though each group called its god or gods the lord of the world.
Christianity started as a faith for Jews who dissented from established Jewish beliefs and practices. But it broke away from identifying with one particular people. This was a new concept, I think.
I’m not sure how it happened. Maybe Christianity spread beyond national and tribal boundaries because its message was universal rather than tied to a particular people. Or maybe Christianity became universal because of the rise of new, widespread transportation networks like the Silk Road and the Roman roads; earlier faiths began and developed in one place among one people, but Christianity spent its early years in a more interconnected, cosmopolitan world.
Eventually, of course, national and regional populations developed their own Christianities: Greek Orthodox, Russian Orthodox, Church of England (Anglican), Southern Baptist, and so on. But all of them identified as Christian.
Anyway, am I wrong? Was Christianity the first Western faith that wasn’t tied to national, ethnic, or tribal identity?