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Arnaud Amaury or Arnaud Amalric (died 1225) was a Cistercian abbot who took a prominent role in the Albigensian Crusade. He is most remembered for allegedly advising a soldier, who was worrying about killing orthodox Catholics along with the heretics during the sack of Cathar stronghold of Béziers, Caedite eos. Novit enim Dominus qui sunt eius. ("Kill them. For the Lord knew those that are His own.")
Early life 
Albigensian Crusade 
In 1204, he was named a papal legate and inquisitor and was sent by Innocent III with Peter of Castelnau and Arnoul to attempt the conversion of the Albigensians. Failing in this, he distinguished himself by the zeal with which he incited men by his preaching to the crusade against them. He was in charge of the crusader army that sacked Béziers in 1209. There, according to the Cistercian writer Caesar of Heisterbach, Arnaul Amalric supposedly responded when asked by a Crusader how to distinguish the Cathars from the Catholics,
Caedite eos. Novit enim Dominus qui sunt eius (Kill them all. For the Lord knoweth them that are His.).
This is the origin of the modern phrase, "Kill them all and let God sort them out."
On the other hand, the legate's own statement, in a letter to the Pope in August 1209 (col.139), states:
While discussions were still going on with the barons about the release of those in the city who were deemed to be Catholics, the servants and other persons of low rank and unarmed attacked the city without waiting for orders from their leaders. To our amazement, crying "to arms, to arms!", within the space of two or three hours they crossed the ditches and the walls and Béziers was taken. Our men spared no one, irrespective of rank, sex or age, and put to the sword almost 20,000 people. After this great slaughter the whole city was despoiled and burnt...
As a matter of fact, Caesar did not state that this sentence had been actually uttered: more exactly he just wrote that Amalric was reported to have said it (dixisse fertur in the original text).
Later life 
According to Moréri, Arnaud was named archbishop of Narbonne about 1212, after his return from an expedition into Spain to encourage the Christians against the Moors. He left an account of this expedition. His stirring spirit embroiled him with his sovereign, Simon de Montfort. In 1224, he presided in the council of Montpellier, assembled to consider the complaints of the Albigensians.
- Tugwell, Simon. Early Dominicans. Paulist Press. ISBN 0-8091-2414-9. 1982. p 114-115.
- MD Costen. The Cathars and the Albigensian Crusade. Manchester University Press 1997. p. 121. ISBN 0-7190-4331-X.
- "Dialogus Miraculorum - Page 308". AHOM (in French).
- Albigensian Crusade
- Meschini, Marco (2010). L'eretica - Storia della crociata contro gli Albigesi. Laterza. ISBN 978-88-420-9306-0. p. 116
- Rose, Hugh James (1857). A New General Biographical Dictionary, London: B. Fellowes et al.