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,