That quote is from the Crusades.
Not just any Crusade but the Albigensian Crusade in southern France, northern Spain against the Cathars.
Up to a million (by some reports) killed in a n attempt by Christian authorities to expunge the cathar heresy.
The actual reported phrase was “Caedite eos. Novit enim Dominus qui sunt eius” which means “Kill them. For the Lord knows those who are his”.