You wrote:
“I think the notion of killing heretics, as in the crusade against the Cathars was a heresy imported from Islam.”
False. Christians - and secular states like the Roman Empire - were killing violent heretics before Islam existed.
“A military leader asked how he could determine if a Cathar was a heretic. “Neco es omnes, Dio se agnoscet”. was reportedly the response that the learned monk gave. Kill them all, G-d knows his own.”
Actually, no. You made two mistakes: 1) Your first mistake is believing it ever happened. No contemporary source supports the story. Caesarius of Heisterbach probably invented the quote for dramatic effect. 2) You also have the quote wrong. It is “Caedite eos. Novit enim Dominus qui sunt eius.” http://gahom.ehess.fr/relex/dialogusmiraculorum1/CdH-Dialog.mir.-Vol1/CdH-Dialog.mir.-Volume1-308.html Even the version of the quote you use is odd in that it appears in several different spellings on the internet. That leads me to believe it is made up, phony quote made up by people sitting online with a Cassel’s dictionary and little knowledge about history. Don’t be fooled.
The Fathers of the Church wrote urgently against killing heretics. All of the great heresiarchs condemned by the Ecumenical Councils died in exile, rather than being executed for heresy (though admittedly some received very harsh treatment during their exile), as did some Fathers of the Church, accused of heresy by heretics (as St. John Chrysostom). It is also true that burning was the prescribed method of execution for heretics from very early on -- but that meant, until contact with Islam created the idea in the West, later, alas, copied in Russia, that heresy per se should be a capital offense, contrary to the teachings of the Fathers -- that heretics found guilty of capital crimes were executed by burning, rather than beheading or hanging. (Oddly to modern sensibilities this was considered a merciful act since the hope was that dying in a foretaste of hell-fire, the heretic might in death abjure his heresy and be saved.)
Can you find any well-attested instances of Christian heretics being executed solely on a charge of heresy, without another capital charge as the basis for the execution, prior to the rise of Islam, and with the approval of the Church? I am aware of none.
The view suggested by donmeaker -- that the Western treatment of heresy as a capital offense is a corruption imported from Islam -- certainly has current scholarly support, cf. Emmett Scott's Mohammed and Charlemagne Revisited. Scott also notes a shift in the attitude toward witchcraft after contact with Islam: before that the Church, uniformly, East and West, taught that the notions that witches flew through the air, and the like, were pagan superstitions unworthy of belief by Christians; after, in the West, if one didn't believe nonsense about witches, which largely paralleled Muslim ideas, one was condemned as heretical.
***was reportedly the response that the learned monk gave. Kill them all, G-d knows his own.”***
****That leads me to believe it is made up, phony quote made up by people sitting online with a Cassel’s dictionary***
I remember reading that quote many year before there was an internet.