I once heard the dictum that “Islam conquers where Christianity has gone into heresy.”
The Syriac and Arabic Christians had rejected the Council of Chalcedon (451) and gone into schism from Rome and Constantinople.
Northern African “Christianity” was predominantly Arian, Pelagian, etc.
Islam swept through those regions like wildfire but was always staved off by the Roman and Byzantine Churches. After the Great Schism, Islam overwhelmed the Byzantine Empire.
Now, here’s the thought that occurred to me, in terms of the old Constantinople versus Rome debate. For the next several centuries, Europe was saved from Islamic incursions by Catholic prayers, Lepanto being the most famous example.
So, here’s my thought: if, in the Great Schism, Constantinople had been “right,” then Rome would have fallen, the Hagia Sophia would be the center of Christianity and the Vatican would be one of the biggest mosques in the world.