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 '' . .
sacked Béziers
Massacre at Béziers
Arnaud Amalric