The Kingdom of Axum I thought was Jewish at first - then converted. Obviously quite a while ago.
I know though that the Jewish Ethiopians are considered by Historians as more Jewish than the Haredi Jews (The ultra orthodox jews) and celebrate rituals older and considered more original than any of those still practiced in Israel.
Aksumites practiced a polytheistic religion before the conversion to Christianity around 320-330 CE. When Ezana was crowned as the emperor, he declared the kingdom to be Christian. The name Ethiopia didn't appear until a bit later still.
Beta Israel was actually formed as a rebellion against the new Christian emperor after he declared the kingdom to be a Christian state. The modern-day Beta Israel population in Ethiopia is actually descendant from local Ethiopians, Most of the Beta Israel have relocated to Israel, though.