A convenient starting point for the history of dualist heresy in medieval Europe is R. I. Moore's The War on Heresy: Faith and Power in Medieval Europe (London, 2012). Moore leaves the attentive reader in no doubt that dualist heresy flourished in the thirteenth century. It is worth collecting some of the data he provides together, since one or two reviewers appear to have missed it. (How did they do so? The key passages occur near the end of the book, by which time it would be easy for a reader to have decided what its central argument was and to miss Moore's conscientious record of evidence that complicates the overall picture, especially since the central thrust of the argument is foregrounded and the complexities are fitted in smoothly and quite unobtrusively, as in an Economist article.) Writing about an inquisition in 1245–6, Moore comments that:
Dualism is certainly suggested by occasional comments incidental to the inquisitors’ immediate concerns – that God did not make the world, that the devil did, that a man who slept with his wife could not be saved (and so might just as well sleep with somebody else.)
Testimony presented […] by a group of Franciscan friars [in 1247] […] was more revealing. […] The unique value of the friars’ stories […] is that they record a spontaneous account of Pier's beliefs, given of his own volition. The God who had given the law to Moses, he said, was a malevolent scoundrel […] Marriage was prostitution, and nobody who slept with a woman, even his own wife, could be saved.
Turning from southern France to Italy, Moore gives a good deal of space to the inquisitorial treatise written c. 1250 by Rainier Sacconi, ‘who had been a Cathar for seventeen years and occupied a leading position among them […] before joining the Dominicans’ (p. 315) – and so presumably knew what he was talking about. He quotes Sacconi's words: ‘All Cathars believe that the devil made the world and everything in it […] they regard as mortal sins reproductive sex, the consumption of its fruits, meat, eggs and cheese’ (pp. 315–16).