>>211919786>it was unthinkable before the 19th centuryIt wasn't. Writers from Nicaea in the 13th century to subscribe to a Greek identity over a Roman one. It just wasn't widespread outside of elite circles.
>And everyone belonging to the Roman church was a Roman, it wasn't an ethnic identity, but a religious one.No it really wasn't. The only reading of it I have ever found was from St. Patrick who uses the term liberally but another Briton, Gildas, basically states outright that they are not Roman and that Romans are foreigners to them despite their shared faith. The continent had even harder lines between ethnic groups with ethnic laws in place affecting Romans. For example in Frankish and Visigothic law, there were differences in how law was administered for a Roman, or a Frank, or a Goth and so on. All while sharing the same faith. People living in the Later Roman Empire had a pretty strong idea of what it meant to be a Roman and it was tied up in an ethnic one. This is even more true in Byzantium, Constantine VII mentions converted pagans and mentions they still call themselves Hellene to his day, and his complaint isn't about religion but rather their ancestry, saying that their ancestors were Romans and not Greek and so shouldn't call themselves as such.