“ISLAM has differed from all the other heresies in two main points which must be carefully noticed:
(1) It did not rise within the Church, that is within the frontiers of our civilization. Its heresiarch was not a man originally Catholic who led away Catholic followers by his novel doctrine as did Arius or Calvin. He was an outsider born a pagan, living among pagans, and never baptized.
. . . (2) This body of Islam attacking Christendom from within, happened to be continually recruited with fighting material of the strongest kind and drafted in from the pagan outer darkness.”
~Hilaire Belloc: The Great Heresies.