My understanding is that a French General, when asked by his lieutenants how to separate the Muslims from the Christians during the conquest of a city, were told , kill them all for God will know his own.
I believe it was Abbot Arnaud-Amaury...
Of course the paradox here is that the origins of the slogan have to do with killing Christians along with Muslims.
what is a french general?
Amaury was a clergyman, not a general (though, because he was the church's representative on the crusade, he was nominally in charge of the army). The Albigensians were not Muslim; they were sort-of "Christians" (though only by their heritage... their religion actually resembled a more dualistic Manichaean sort of religion with a little Donatism thrown in).