To add to the other comment, a lot of Middle Eastern Christians were (and still are) Nestorian and Miaphysite Christians, rather than Chalcedonian, which is what the Eastern Orthodox and Roman Catholic Churches are often called before their schism in 1054.

Nestorianism, today represented by the Assyrian Church of the East, is the belief that Jesus Christ was two distinct persons, one divine and one human. It was condemned as heretical at the Council of Ephesus in 451 and many Nestorians fled Byzantine lands to the Sassanid Empire, because the largely Zoroastrian Sassanids were not interested in enforcing Christian orthodoxy.

Miaphysitism, today represented by the Copts, Armenian Apostolic Church, and the Syria Orthdoox Church, is the belief that Jesus Christ was both fully human and fully divine, in one nature.

The Chalcedonians (which today comprises Eastern Orthodox, Roman Catholics, and most Protestants) take the position that Jesus was one person with two natures, divine and human.

This differences might seem small, but they led to a lot of bloodshed in Western Asia and North Africa in the 5th and 6th centuries. Further hardening the differences is that the Chalcedonians believed that Greek and Latin were the only acceptable liturgical languages, whereas a number of Nestorian and Miaphysite Christians used Coptic, Assyrian, Armenian, or Syriac instead.

It is worth noting that a number of Byzantine writers tended to assume that Islam was a form of Nestorian or Miaphysite Christianity as did the Catholic chronicler, Guillaume of Tyr.