[The Crusades] were not the brainchild of an ambitious pope or rapacious knights but a response to more than four centuries of conquests in which Muslims had already captured two-thirds of the old Christian world. At some point, Christianity as a faith and a culture had to defend itself or be subsumed by Islam. The Crusades were that defense.
— Madden, Thomas F. Crisis. 4 Apr. 2002.