Arnaud Amaury or Arnaud Amalric was a Cistercian Abbot who was in charge of the Crusader army that sacked Béziers in the Albigensian Crusade 1209 to 1229. He distinguished himself by the zeal with which he incited men by his preaching to crusade against the enemy. Arnaul Amalric is most remembered for allegedly advising a Crusader how to distinguish the Cathars from the Catholics because the soldier was worried about killing orthodox Catholics along with the heretics during the sack of the Cathar stronghold of Béziers. Arnaud Amalric responded, “Caedite eos. Novit enim Dominus qui sunt eius.” (“Kill them all. For the Lord knows those that are His.”)
This is the origin of the modern phrase, “Kill them all and let God sort them out.”