In Toulouse in 1761, a shopkeeper’s son hanged himself in the family home. A rumour quickly went round the town that the son had been killed because he wanted to become a Catholic. The shopkeeper, Jean Calas, was a Huguenot, and the town was about to celebrate – and “celebrate” is the word – the 200th anniversary of the Saint Bartholomew’s Day massacre, when 2,000 Protestants were murdered, with maximum barbarity, by the town’s Catholics. So the Toulousains were in no mood for tolerance, or even scepticism (such as to how a frail man in his 60s could kill a large, healthy man some 40 years younger). Calas was tortured, and then executed.

This was the incident that propelled Voltaire into writing his Traité sur la Tolérance, an examination into the long-established tradition of murdering people who go against dogma; specifically religious dogma (although of course there are other dogmas these days). Voltaire knew whereof he spoke: he was by this time living at Les Délices, near Switzerland, and he would periodically hop over the border whenever the French authorities became enraged by what he wrote.

What particularly bothered him was that it was generally considered to be the age of reason. Of the Calas case, he said: “It seems as if fanaticism, which had been affronted by the success of reason in the recent past, struggled even more angrily under the yoke of rationality” – words that should ring down through the centuries. If we take one thing from Voltaire it should be a warning against our complacency.

Not that anyone who is keeping half an eye on events is complacent. The Traité became a bestseller again in France two and a half centuries after its publication, when 11 people were murdered in the Paris office of Charlie Hebdo magazine in 2015. It is difficult to pin down which section of this book would be the most pertinent to that awful event: perhaps the part where Voltaire speculates whether someone who murders 4,000 Protestants is twice as holy as someone who murders 2,000; or his imagined speech to “an imam or a Buddist monk”: “Listen to me, because the God of all the worlds has enlightened me; there are 900 million little ants like us on Earth, but there is only one anthill that is cherished by God. He has hated all the others since the beginning of time”, etc. He imagines the reaction from his audience. “I would try to calm them, but it would be very difficult.” It would indeed, for, as he said in his next work, the Philosophical Dictionary, “Lorsqu’une fois le fanatisme a gangrené un cerveau, la maladie est presque incurable.” That is: “Once fanaticism has gangrenated the brain, the sickness is almost incurable.” (There is such a word as “gangrenate”, in case you were wondering, but it is considered obsolete, for reasons I cannot fathom.)

Still, as long as this work is around, there is hope. That it has been issued in this edition by Penguin is a cause for celebration, and it should go on your shelves as soon as possible. This fluent translation by Desmond M Clarke, who died last month, comes with a very good introduction and copious notes to help us through what might otherwise have been obscure references to various religious controversies in 18th-century France.

And in case the example of this work does not promote the tolerance it hopes for, there is always this approach, as given in an interlude in which Voltaire tells a fable about a Jesuit, a Danish and a Dutch chaplain in China arguing ferociously among themselves: a mandarin, disturbed by the noise, asks for an explanation, and when they continue fighting he throws them all in prison until they agree. They will never agree, he is told. Well, they must forgive each other, he says. That, he is told, is also impossible. In which case, he says, “they must remain in prison until they pretend to forgive each other”.

And he leaves it at that.