Of the external foes of the Church, Mohammedanism was one of the most dangerous and long enduring. Ideologically it was not external, but one of the great heresies, containing as it did many Christian elements, mingled with some borrowed from Judaism, and much gross and carnal materialism foreign to both…But the organization of Islam was outside the Christian fold, and its pressure was from without. In its very first century of existence, this sect conquered not only large tracts of Asia, but all northern Africa as far as the Atlantic, engulfed Spain, and invaded France, leaving there, as on the crest of a highest wave, marks of its own culture which persisted even to modern times. Nor did the danger of a Mohammedan conquest of all Europe end with the Battle of Tours in 732. The peril endured for nearly a thousand years, now menacing France, now Italy, now striking through the Balkans, now through northeastern Europe: now on land, now on sea. In the tenth century the Moslems held Sicily, whence they raided the mainland of Italy, laid waste the country about Rome, profaned the apostolic tombs, and collected tribute from the Popes. They had been a threat to the peace and independence of European Christians for nearly four centuries when Pope Urban II preached the First Crusade against them in 1096.
William Thomas Walsh, Characters of the Inquisition (1940)