Publishers are rushing to reprint Voltaire’s Treatise on Tolerance (1763) following a run on copies in the week after the deaths of 12 people in the attack on the offices of Charlie Hebdo, according to French publisher Gallimard. Written after the execution for murder of Protestant merchant Jean Calas, championed by Voltaire as a wrongly convicted victim of Catholic persecution, the treatise has also doing well on amazon.fr, with editions including a free Kindle ebook and a €2 paperback the No 1 bestsellers in religion, philosophy and other categories.

Two hundred and fifty years after Voltaire’s campaign resulted in Calas being posthumously declared innocent, the protean author’s name and image were hard to avoid in the wake of the Charlie Hebdo murders. It was down Paris’s Boulevard Voltaire that record numbers (including 44 world leaders) marched last weekend, and it was a portrait of him that the Palace of Versailles put on display in tribute to the jihadists’ victims.

A poster showing Voltaire’s bust with a je suis Charlie black band (pictured) illustrated articles and appeared on social media, and he was name-checked by fans as varied as Boris Johnson and Banksy. The austere keepers of the flame of the Société Voltaire argued that “it was also Voltaire that [the killers] wanted to assassinate”, meaning the Enlightenment legacy of sceptical rationality, laicité (secularism), free-ranging curiosity and battles against censorship that he embodied; but so too did a leader in glossy, celebrity-fixated Paris Match.

Voltaire’s supposed words “I disapprove of what you say, but I will defend to the death your right to say it” – in relation to the burning of On Mind by the atheist philosopher Helvetius, a book he privately dismissed as a trivial “omelette” – went viral, but even those tweeting them often knew that they were only attributed to him (by, bizarrely, an Edwardian British biographer, Evelyn Hall, whose attempted paraphrase came to be treated as a real quotation).

Awareness of this dodgy provenance, and of the words having been overused, perhaps contributed to a subsequent unease about deploying Voltaire as a touchstone for free expression. Relevant thoughts on other topics were found instead, such as his condemnation of fanaticism, and the Charlie Hebdo-Voltaire equation was questioned – his authentic, published quotes on censorship tend to be about written arguments, leaving unclear his views on provocative images that can’t be “refuted”. Journalists and ministers who had shown support for Charlie Hebdo’s right to offend were denounced not only as fake Voltaireans, but as people he probably would have despised. The papers declined to reproduce the magazine’s cartoons of Muhammad, and the marching politicians were simultaneously pushing through laws against hate speech (French prime minister Manuel Valls) or extending police powers in a way likely to crush investigative journalism or promoting increased surveillance (David Cameron). Like George Orwell, Voltaire is an awkward bedfellow, intolerant of hypocrisy and stupidity as well as zealotry, and always liable to turn his ironic gaze on you.