Islam, a Great Heresy

"THE MOHAMMEDAN attack was a different kind. It came geographically 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 was a heresy alien rather than intimate. It threatened to kill the Christian Church by invasion rather than undermine it from within."

~Hilaire Belloc: "The Great Heresies."