If one studies in fact the progress of the primitive Christian missions, he remarks that they are all directed towards the West: in other words, they take the Roman Empire as their theatre and limit. If one excepts some small portions of the vassal territory of the Arsacidæ, lying between the Euphrates and the Tigris, the empire of the Parthians received no Christian missions during the first century. The Tigris was an eastern boundary which Christianity did not pass under the Sassanidæ. Two great causes--the Mediterranean and the Roman Empire--determined this capital fact. The Mediterranean had been, during a thousand years, the great route on which all civilizations and all ideas had passed each other. The Romans, having freed it from piracy, had made it an unequalled way of communication. It was in a sense the railroad of that time. A numerous marine of coasting-vessels rendered the voyages along the borders of this great lake very easy. The relative security which the routes of the empire afforded, the sure guaranties found in the public powers, the scattering of the Jews over all the coasts of the Mediterranean, the use of the Greek tongue in the eastern portion of this sea, the unity of civilization which the Greeks first, and then the Romans, had created, made the map of the empire also the map of the countries reserved to the Christian missions and destined to become Christian. The Roman _orbis_ became the Christian _orbis_ in the sense in which it may be said that the founders of the empire were the founders of the Christian monarchy, or, at least, that they have drawn its outlines. Every province conquered by the Roman Empire became a province conquered by Christianity. Let the figures of the apostles be imagined in the presence of Asia Minor, of Greece, of Italy divided into a hundred little republics, of Gaul, of Spain, of Africa, of Egypt, with its old national institutions, and their success can no more be thought of, or rather it would seem that their project could never have had birth. The union of the empire was the necessary preliminary condition of all great religious propagandism, placing it above nationalities. The empire recognized this in the fourth century. It became Christian. It saw that Christianity was the religion which it had accepted without knowing it,--the religion limited by its frontiers, identified with it, capable of bringing it a second life. The Church, on its side, made itself entirely Roman, and has remained to this day a fragment of the empire. During the middle ages the Church was the old Rome, seizing again its authority over the barbarians, imposing on them its decretals, as formerly it had imposed its laws, governing them by its cardinals, as it had before governed through its imperial legates and proconsuls.