Arnaud Amaury (Latin: Arnoldus Amalricus; died 1225) was a Cistercian abbot who played a prominent role in the Albigensian Crusade. Prior to the massacre of Béziers, it was reported that Amalric, when asked how to distinguish Cathars from Catholics, responded, "Kill them all! God will know his own."
In 1204, he was named a papal legate and inquisitor and was sent by Pope 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. For the Lord knows who 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). There is little if any doubt that these words captured the spirit of the assault, and that Arnaud and his crusaders planned to kill the inhabitants of any stronghold that offered resistance. The crusaders (which Arnaud referred to as nostri, "our men") rampaged and killed without restraint.
Arnaud himself, in a letter to the Pope in August 1209 (col.139), wrote:
...dum tractatetur cum baronibus de liberatione illorum qui in civitate ipsa catholici censebantur, ribaldi et alii viles et inermes personæ, non exspectato mandato principum, in civitatem fecerunt insultum, et mirantibus nostris, cum clamaretur : Ad arma, ad arma, quasi sub duarum vel trium horarum spatio, transcensis fossatis ac muro, capta est civitas Biterrensis, nostrique non parcentes ordini, sexui, vel ætati, fere viginti millia hominum in ore gladii peremerunt; factaque hostium strage permaxima, spoliata est tota civitas et succensa...
...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...
Both Arnaud and Caesarius were Cistercians, and Arnaud was the head of the Cistercian Order when Caesarius wrote. Caesarius required an imprimatur, so it is unlikely that Arnaud's alleged order as reported by Caesarius was seen at the time as reflecting badly on Arnaud. On the contrary the incident was included as an exemplum in Caesarius's Book of Miracles because (to Cistercians at least) it reflected well on Arnaud.
After helping the Crusaders capture Carcassonne, Arnaud was replaced as commander of the army by Simon de Montfort, 5th Earl of Leicester. However, he continued to accompany the men and to exercise significant authority.  On July 22, the Siege of Minerve concluded when the town's defenders agreed to surrender. Simon and the commander of the defenders, Guilhem de Minerve agreed to terms of surrender. However, Amalric, who had been absent at the time, returned to camp. He insisted that no agreements could be considered binding without the assent of himself as papal legate. Simon wished to treat the occupants leniently, but Amalric wanted them put to death. Eventually, the two worked out a solution. The Crusaders allowed the soldiers defending the town, as well as the Catholics inside of it, to leave. The Cathars who had not yet reached the status of perfect were also allowed to go free. The Cathar perfects were given the choice to return to Catholicism or face death. This solution angered many of the soldiers, who had wanted to participate in a massacre. Amalric calmed them by insisting that the majority of perfects would not recant. His prediction was correct. Only three women recanted. The remaining 140 were burned at the stake.
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.
- Costen 1997, p. 121.
- "Dialogus Miraculorum - Page 308". AHOM (in French). Archived from the original on 2012-02-20.
- Meschini, Marco (2010). L'eretica - Storia della crociata contro gli Albigesi. Laterza. ISBN 978-88-420-9306-0. p. 116
- Jacoby, Russell (2011). Bloodlust: On the Roots of Violence from Cain and Abel to the Present. Free Press. p. 29f. ISBN 978-1-4391-0024-0. OCLC 787862175.
Caesarius of Heisterbach amalric.
- William of Tudela, cited in Zoé Oldenburg, Massacre at Montségur, page 116
- Sibly, W.A.; Sibly, M.D. (2003). The Chronicle of William of Puylaurens: The Albigensian Crusade and Its Aftermath. Woodbridge, Suffolk, England, UK: The Boydell Press. pp. 127–128. ISBN 9780851159256.
- Oldenbourg, Zoé (2000). Massacre at Montségur: A History of the Albigension Crusade. Phoenix Press. p. 109ff. ISBN 1-84212-428-5. OCLC 47720027.
- Albigensian Crusade
- McDonald, James (2021). Kill Them All! Did a Medieval Abbot give this command to his Crusader Troops?. ACHS. ISBN 979-8598792780.
- Peter of les Vaux de Cernay 1998, pp. 101.
- Marvin 2009, p. 77.
- J.C.L. Simonde de Sismondi 1973, p. 63.
- Strayer 1971, p. 71.
- Peter of les Vaux de Cernay 1998, pp. 156.
- Rose, Hugh James (1857). "Amalric, Arnaud". A New General Biographical Dictionary. 1 AA–ANS. London: B. Fellowes et al. p. 376.
- Gracias, Luke, "The Devil's Prayer, Australian eBook Publishers 2016, ISBN 9781925427332
- Costen, Michael D. (1997). The Cathars and the Albigensian Crusade. Manchester and New York: Manchester University Press. ISBN 0-7190-4331-X.
- Marvin, Laurence W. (2009). The Occitan War: A Military and Political History of the Albigensian Crusade, 1209-1218. Cambridge, UK: Cambridge University Press. ISBN 978-0521123655.
- J.C.L. Simonde de Sismondi (1973) . History of the Crusades Against the Albigenses in the Thirteenth Century. New York, NY: AMS Press.
- Strayer, Joseph R. (1971). The Albigensian Crusades. New York, NY: The Dial Press. ISBN 0-472-09476-9.