Kill them all...

I am trying to find the origin of this phrase or quote. Please help me if possible.
The phrase is "Kill them all, let God sort them out". I have been told it has to do with the sacking of a city that had Christian inhabitants.
I am trying to write a reply to a letter to the editor and would like some quick help please and thank-you.

A historical explanation involving a real city seems unlikely. Can your informant document that idea? I suspect that the phrase originated in 20th-century popular culture.

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