We Americans, and indeed, most Europeans, nowadays, have trouble with the idea that some people don’t, and have never, had any idea of “separation of Church and State”. In fact, for most of history among most of mankind, one of the first things a new government took care to take over was the religious practice of its people. We are generally aghast at the misdeeds of ISIS in the Middle East and the Taliban in Afghanistan, for destroying historically important shrines and monuments. Actually, we shouldn’t be surprised. It’s exactly what most conquerors have always done. Read the Old Testament: it’s what the “Children of Israel” did when they entered the Holy Land. It’s only unusual to us because we’ve always done it differently.
That’s at least part of the reason that the history of the Christian Church is so awfully interesting. Before being legalized in 312, only two or three governments in the world did not persecute Christians, at least occasionally. After 312, the Church was tolerated (and later enforced, itself) within the Roman Empire, but that left a lot of the world where it was not. Within the Empire, though, Christianity became not only permitted, but required. Beginning right then, though, things began to change.
Well before 400, the Emperor ruled not from Rome, but from Constantinople. His plan was to rule through the Patriarchs, and that worked pretty well in Greece and the old Eastern Roman Empire. The Patriarch of Constantinople, often called “The Ecumenical Patriarch” was titular head of the whole Church, but none of them ever asserted any separation from the Emperor. The Patriarchs of Jerusalem, Antioch and Alexandria were also subject to the Emperors, but in reality largely independent until the 600s, when Mohammed and his Arabs came on the scene and captured the lands that they were supposed to be in charge of. Islam is technically the Christian heresy of “Monarchianism”, but Muslims get very irritated when you tell them that. In any case, between 700 or so and 1450 or so, Islam and Constantinople fought pretty much constantly, but both agreed completely that the prince in charge of the people got to decide how everyone worshipped. In both cases, Church and State were one, with the civilian ruler in charge of both. Only in the West did it work out differently. There, in the old Western Roman Empire, there was no civil ruler of any consequence, only a horde of barbarian Germanic chieftains, running rampant over the old civilized provinces of the Empire.