The first thing that needs to be understood is this: The Roman Empire was a united civilization, the prime characteristic of which was the absolute and unconditional acceptance of one common mode of life by everyone who dwelt within its boundaries.
It’s a difficult concept for a modern person to grasp. When we think of Europe, we are, the European Union notwithstanding, still accustomed to think of a number of sovereign countries, more or less sharply differentiated, and each often colored by different customs, language, and religions: France and its French language, with romance and peasant villages. Germany and its German language, with stern discipline and beer. Italy and its Italian language, with swarthier folk and wine. European nations are, in a way, defined by what they aren’t when compared to the their neighbors.
But people living in the Roman Empire regarded civic life in a totally different way. All conceivable antagonisms (and they were violent) were antagonisms within one State. From the Euphrates to the Scottish Highlands, from the North Sea to the Sahara and the Middle Nile, all was one State.
The world outside the Roman Empire was considered a sort of waste: not thickly populated, no appreciable arts or science. Barbaric. Sure, they were often a menace to the Empire’s frontiers, but that menace was seen as an irritation: the hassle of preventing a fringe of imperfect, predatory, and small barbaric communities from harming the vast, rich, thickly-populated, and highly-organized State within those frontiers.
Moreover, those barbarians–Germanic and Slavs east of the Rhine and north of the Danube, nomads of the desert, Irish and Picts–were themselves tinged by the Empire. Roman trade permeated their cultures. We find Roman coins everywhere. Latin and Greek terminology permeated their speech. They admired it. They perpetually begged for admittance.
But they never dreamed of “conquest.”
The Roman administrator, on the other hand, tried to get the barbarians to settle on the frontier fields, so he could exploit their labor, acts as a buffer against barbarians from further away that weren’t as colored by Roman ways, and serve as mercenaries in Roman armies. So they were useful but still, overall, insignificant in the eyes of the Empire.
The only exception to this was the small common frontier with Persia. Persia was its own civilization and was more powerful than the barbarians or nomads (“The Persian War” was the only serious foreign war in the eyes of all rulers from Julius Caesar to the sixth century). The frontier line with Persia often shifted, but the Mediterranean peoples of the Levant, from Antioch to Judea, were always Roman and within the Empire. Overall, the strife with Persia counted little in the general life of Rome.
Anyway, regardless of Persians and some occasional barbarian troubles, the central point to grasp here is that, during the first centuries of the Christian era, people lived as citizens of one State. It was a State they took for granted and that they even regarded as eternal.
Sure, people grumbled about taxes and there were occasional revolts, but there was never a suggestion that someone else besides the imperial authority should be levying the taxes and collecting them. There was plenty of conflict between armies and individuals over control, but no one doubted that someone–the Emperor–should despotically control. Localities had their own customs, but no one conceived them to be antagonistic to the one State. That State was, for the people within it, the World.
The complete unity of this social system was even more striking when we look at the innumerable local customs and liberties, as well as the dizzying variety of philosophic opinions, religious practices, and dialects within that world. There wasn’t even an official language; there were two, Greek and Latin.
But despite the mosaic of underlying local customs and practices, the power of Emperor–whether held by one man or four, whether held by a wise man or a tyrant–was always one power, one office, and one system.