It might have appeared to any man watching affairs in the earlier years of the seventh century – say from 600 to 630 – that only one great main assault having been made against the Church, Arianism and its derivatives, that assault having been repelled and the Faith having won its victory, it was now secure for an indefinite time.
Christendom would have to fight for its life, of course, against outward unchristian things, that is, against Paganism. The nature worshipers of the high Persian civilization to the east would attack us in arms and try to overwhelm us. The savage paganism of barbaric tribes, Scandinavian, German, Slav and Mongol, in the north and centre of Europe would also attack Christendom and try to destroy it. The populations subject to Byzantium would continue to parade heretical views as a label for their grievances. But the main effort of heresy, at least, had failed – so it seemed. Its object, the undoing of a united Catholic civilization, had been missed. The rise of no major heresy need henceforth be feared, still less the consequent disruption of Christendom. . . .
It was just at this moment, a moment of apparently universal and permanent Catholicism, that there fell an unexpected blow of overwhelming magnitude and force. Islam arose – quite suddenly. It came out of the desert and overwhelmed half our civilization. . . .
[Islam ] was a heresy: that is the essential point to grasp before going any further. It began as a heresy, not as a new religion. It was not a pagan contrast with the Church; it was not an alien enemy. It was a perversion of Christian doctrine. Its vitality and endurance soon gave it the appearance of a new religion, but those who were contemporary with its rise saw it for what it was – not a denial, but an adaptation and a misuse, of the Christian thing. It differed from most (not from all) heresies in this, that it did not arise within the bounds of the Christian Church. The chief heresiarch, Mohammed himself, was not, like most heresiarchs, a man of Catholic birth and doctrine to begin with. He sprang from pagans. But that which he taught was in the main Catholic doctrine, oversimplified. It was the great Catholic world – on the frontiers of which he lived, whose influence was all around him and whose territories he had known by travel – which inspired his convictions. He came of, and mixed with, the degraded idolaters of the Arabian wilderness, the conquest of which had never seemed worth the Romans’ while.