Arnaud Amaury (Latin: Arnoldus Amalricus; died 1225) was a Roman Catholic 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." Whether this was actually said is sometimes considered doubtful.
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). 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...
After helping the Crusaders capture Carcassonne, he was replaced as commander of the army by Simon de Montfort, 5th Earl of Leicester. However, he continued to accompany the men.  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.
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