Arnaud Amalric (died 1225) was a Cistercian abbot who took a prominent role in the Albigensian Crusade. He is remembered for supposedly advising a soldier, who was worried about killing orthodox Catholics instead of just heretic Cathars prior to the sack of Béziers, to simply kill everyone as God would sort it all out later.
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 Caesarius of Heisterbach, Arnaud Amalric 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."
Caesarius did not hear this statement first hand, merely writing that Arnaud was reported to have said it (dixisse fertur in the original text). Arnaud himself, in a letter to the Pope in August 1209 (col.139), wrote:
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...
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.
Very little is known about the life of Arnaud Amalric after 1222. History justified his absence by noting that, contrary to expectations, he took no part in the ongoing quarrels between the sons of Montfort and Raymond. On 29 September 1225, Arnaud Amalric died in Fontfroide, France. 
- 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).
- Meschini, Marco (2010). L'eretica - Storia della crociata contro gli Albigesi. Laterza. ISBN 978-88-420-9306-0. p. 116
- Albigensian Crusade
- Rose, Hugh James (1857). A New General Biographical Dictionary, London: B. Fellowes et al.
- Gracias, Luke, "The Devil's Prayer, Australian eBook Publishers 2016, ISBN 9781925427332