[wahern]

The Church of the East split in the early 5th century, followed later that century by the Oriental Orthodox Churches. Altogether they may have been larger than the Roman (Latin and Greek/Byzantine) Church.

Armenia became a Christian state before the Roman Empire did, and Ethiopia not long after Rome did. (Both churches now part of Oriental Orthodoxy. Another state church around the area of Sudan emerged, too, the last of whom disappeared only in the 1600-1700s.)

The growth of Christianity in the Persian Empire created a major source of friction between the Roman and Persian Empires (the latter was nominally Zoroastrian, at that time also a proselytizing religion), creating space for the emergence of Islam, which would later lead to the end of both empires and the conversion of millions of Christians and Zoroastrians.

Christianity was never coterminous with the Roman Empire. It just seemed that way from the perspective of European history and culture. European Christianity eventually forgot about those other Christians (I'm not sure if the reverse was true, though). Relative to modern Protestantism, all of these churches have near identical theology, Roman Catholicism included. Which perhaps bolsters the point about Protestantism representing a significant break in the European historical narrative.