The Great Heresies is possibly the greatest book written by Hilaire Belloc (1870-1953), the famous Catholic historian, for here the author, calling upon his vast knowledge of history, outlines in simple terms for his readers, not only the meaning and influence of heresy against the Catholic Church, but the impact on the entire world of five of the greatest heresies of all time: Arianism, Mohammedanism (Islam), Albigensianism, Protestantism and Modernism.
        "Heresy is the dislocation of some complete and self-supporting scheme by the introduction of a novel denial of some essential part therein. The reason that men [used to] combat heresy is not only, or principally, conservatism—a devotion to routine, a dislike of distrubance in their habits of thought—it is much more a perception that the heresy, in so far as it gains ground, will produce a way of living and a social character at issue with, irritating, and perhaps mortal to [this means it will kill it], the way of living and the social character produced by the old orthodox scheme [e.g. Islam vs. Christianity]."
        "We are what we are today mainly because no one of those heresies finally overset our ancestral religion, but we are also what we are because each of them profoundly affected our fathers for generations, each heresy left behind its traces, and one of them, the great Mohammedan movement, remains to this day in dogmatic force and preponderant over a great fraction of territory which was once wholly ours."
        "Such a study is the easier from the fact that our fathers recognized heresy for what it was, gave it in each case a particular name, subjected it to a definition and therefore to limits, and made its analysis the easier by such definition."
        "The Mohammedan attack was of a different kind. It came geographically from just outside the area of Christendom; it appeared, almost from the outset, as a foreign enemy; yet it was not, strictly speaking, a new religion attacking the old, it was essentially a heresy; but from the circumstances of its birth it was a heresy alien rather that intimate. It threatened to kill the Christian Church by invasion rather than to undermine it from within. Islam very nearly destroyed us. It kept up the battle against Christendom actively for a thousand years, and the story is by no means over; the power of Islam may at any moment re-arise."
        "Mohammedanism 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. That which he taught was in the main Catholic doctrine, oversimplified.
        "Thus the very foundation of his teaching was that prime Catholic doctrine, the unity and omnipotence of God. The attribtes of God he also took over in the main from Catholic doctrine: the personal nature, the all-goodness, the timelessness, the providence of God, His creative power as the origin of all things, and His sustenance of all things by His power alone."
        "But the central point where this new heresy struck home with a mortal blow against Catholic tradition was a full denial of the Incarnation. He taught that our Lord was the greatest of all prophets but still only a prophet: a man like other men. He eliminated the Trinity altogether. With that denial of the Incarnation went the whole sacramental structure. He refused to know anything of the Eucharist, with its Real Presence; he stopped the sacrifice of the Mass, and therefore the institution of a special priesthood. In other words, he, like so many other lesser heresiarchs, founded his heresy on simplification."
        "Millions of modern people of the white civilization—that is, the civilization of Europe and America—have forgotten all about Islam. They have never come in contact with it. They take for granted that it is decaying, and that, anyway, it is just a foreign religion which will not concern them. It is, as a fact, the most formidable and persistent enemy which our civilization has had, and may at any moment become as large a menace in the future as it has been in the past."
        "Will not perhaps the temporal power of Islam return and with it the menace of an armed Mohannedan world which will shake off the domination of Europeans—still nominally Christian [this was written in 1938]—and reappear again as the prime enemy of our civilization? The future always comes as a surprise but political wisdom consists in attempting at least some partial judgement of what that surprise may be. And for my part I cannot but believe that a main unexpected thing of the future is the return of Islam. Since religion is at the root of all political movements and changes and since we have here a very great religion physically paralysed but morally and intensely alive, we are in the presence of an unstable equilibrium which cannot remain permanently unstable."
        "Cultures spring from religions; untimately the vital force which maintains any culture is its philosophy, its attitude toward the universe; the decay of a religion involves the decay of the culture corresponding to it—we see that most clearly in the breakdown of Christendom today."