Muslims recognized Byzantium as a rival universalist faith and empire

Evidence

Immediately to the north were people whom modern scholarship has called Byzantines, but who called themselves and were recognized by their Muslim neighbors as Romans. In this ancient empire, ruled by a line of Caesars and professing the Christian religion, Muslims recognized a state, a faith, and a mission of their own kind—a rival dispensation, maintained by a rival power, disputing with them the possession of God’s final truth and the universal mission of bringing it to all the world. — Bernard Lewis, From Babel to Dragomans, 653

Categorization

• QUESTION: The parallel framing of two universalist ‘dispersions’ (Islam vs. Christianity) each claiming God’s final truth — worth chasing the comparative-universalism angle.