After this great slaughter the whole city was despoiled and burnt ...[3]About thirteen years later Caesarius of Heisterbach relates this story about the massacre, with the papal legate quoted using the words Caedite eos.
The abbot, like the others, was afraid that many, in fear of death, would pretend to be Catholics, and after their departure, would return to their heresy, and is said to have replied "Kill them all for the Lord knoweth them that are His" (2 Tim.
[5]Although Caesarius did not state definitively that this sentence had been uttered, he wrote that Amalric "was reported to have said it" (dixisse fertur in the original text).
[6] There is little if any doubt that these words captured the spirit of the assault,[7] and that Arnaud and his crusaders planned to kill the inhabitants of any stronghold that offered resistance.
Presented with the difficulty of distinguishing Catholics from the Cathars, especially if individuals might misrepresent their own beliefs, the phrase indicated that God would judge those who were killed, and accordingly "sort" them into Heaven or Hell.
[1] The phrase is a partial quotation from the Vulgate version of 2 Timothy 2:19 (Latin: cognovit Dominus qui sunt eius, lit.