The Abbot Arnaud Amaury, head of the Cistercian Order, is most notable for the possibly spurious statement attributed to him during the battle at Beziers, the first and bloodiest combat of the Albigensian Crusade. He reportedly told confused crusaders who could not distinguish between Catholics and Albigensians [Cathars] to "Kill them all. God will know his own." Estimates from the time, including Catholic sources, claim anywhere from 10,000 to 20,000 men women and children were killed in this massacre, although this is certainly an exaggeration as the total population of Beziers was likely no more than eight or nine thousand.(1) As a result, upon hearing of ferocity of the battle, other cities surrendered without resistance.
A paraphrased version of the quote attributed to Arnaud has remained popular into the modern era, especially with military units that have a reputation for fierceness such as the U.S. Marines, Army Rangers, or Special Forces. The paraphrased version is as follows. "Kill'em all and let God sort'em out." This phrase is found posted above doorways, printed on T-shirts sold on military bases, and serves as a sort of unofficial motto for these organizations.