Moses, Jesus and Muhammad: a trio of imposters who gulled ignorant and credulous people into believing impossible things in order to exercise power over them. This was the explosive claim of a treatise that, for more than five centuries, caused both alarm and excitement among Euro-pean churchmen and intellectuals. However, De tribus impostoribus may have been a book that never actually existed – or rather one, as Voltaire famously said about God (ironically in a work denouncing the tract), that it proved necessary to invent. In 1239 Pope Gregory IX accused the mercurial Emperor Frederick II of having composed the work. Over the following centuries numerous other attributions of authorship were made, and occasional sightings of it were reported, though no manuscript survives. Eventually, so Georges Minois suggests, a loose network of heterodox thinkers took it upon themselves to make the myth a reality, and around the start of the 18th century it was printed in distinct Latin and French versions, a compendium of the projected fears of the orthodox and the slogans of the radical Enlightenment.

All of this makes for interesting reading, in a Da Vinci Code-ish kind of way, though the outline of the story has previously been established by detailed scholarly work. Minois, however, wants to use the tale as a peg for hanging up a very large coat: a history of atheism,