Caesarius von Heisterbach

Caesarius of Heisterbach (ca. 1180 – ca. 1240) (sometimes erroneously called in English Caesar of Heisterbach) was the prior of the former Cistercian monastery Heisterbach Abbey, in the Siebengebirge near the little town of Oberdollendorf, Germany.

He is best known as the compiler of a book of hagiography, the Dialogus miraculorum, which is a collection of 746 miracle stories arranged in twelve distinctions. The tales are told in the form of dialogues between a monk and a novice. The work was often referred to by preachers seeking material for sermons in the Late Middle Ages. The work was popular and was widely distributed, showing that it catered well to the tastes of the times; it was perhaps the second largest late mediaeval best-seller, second only to the Golden Legend of Jacobus de Voragine. A vision reported in the book provided the source for the iconography of the Virgin of Mercy.

Caesarius of Heisterbach is also remembered for a maxim on the rise and fall of monasteries: he wrote that discipline causes prosperity in a monastery, and prosperity undermines discipline. He also gave the name of Titivillus as the demon who caused typographical errors in the work of scribes. He is further known as having attributed a famous statement to Arnaud Amalric, a leader in the Albigensian Crusade. Upon being asked how to distinguish Cathars from Catholics at the besieged town of Béziers, Arnaud supposedly replied "Caedite eos. Novit enim Dominus qui sunt eius", which translates as: "Slay them. God knows his own." This statement is often remembered as "Kill them all and let God sort them out."

...
Wikipedia