In case you were wondering, this is a very old quote.
It is attributed to Arnaud-Armaury, the Abbot of Citeaux, and "spiritual advisor" to the Albigensian Crusade.
Pope Innocent III ordered the Albigensian Crusade, to purge southern France of the Cathari heretics. It began in the summer of 1209, with their first target - the town of Beziers. The Catholic faithful in Beziers refused to give up the Catharis among themselves. The crusaders invaded. When Arnaud-Amaury was asked whom to kill he replied "Kill them all. God will know his own." They did. The crusaders slaughtered nearly everyone in town, over 20,000, either burned or clubbed to death. Thus they achieved their goal of killing the estimated 200 heretics who were hiding in the town among the Catholic faithful. The brutal crusade continued on for the next twenty years. Eventually the Catholics devised a new approach for dealing with the remaining Cathari heretics in France. It was called "the Inquisition".
WHO GOT IT RIGHT: Andie J.