IN social terms a value, and in those who practice it a virtue, tolerance nevertheless seems to have been a new idea in the eighteenth century. This may not seem to square with the usual stereotype of that period, which in Europe was supposedly marked by great sophistication in all the arts, lively intellectual curiosity and brilliant social life. But it becomes less surprising when we recall that the wars of religion were then still fresh in everyone's memory, and that fanaticism, albeit in retreat, was not yet dead.
Conversation in the salons was unfettered, but in France many books were forbidden, seized and destroyed. Some unconventional opinions and behaviour were connived at, but that is not to say that they were tolerated: tolerating means accepting the existence of that which is different. Many men tolerant by instinct and conviction were to be found even among princes and potentates, but that did not stop fanaticism from rearing its head under cover of the institutions. Exclusion and persecution stemmed partly from the automatic enforcement of laws, even those privately regarded as outdated, and partly from the demands of a section of public opinion which used minorities as scapegoats for its own frustrations and its taste for violence.
The word tolerance was still viewed with caution, mistrust and sometimes hostility. Though the Encyclopaedists had already had a profound influence in educated circles, there were still theologians ready to defend intolerance--in the most spectacular form it took in France, namely the prohibition of Protestantism by the revocation of the Edict of Nantes in 1685. The negative effects of this measure were still deeply felt several decades afterwards, both in the intellectual sphere (in the form of a brain drain) and in the economy (in the form of an exodus of skilled craftsmen to work abroad).
The advent of tolerance, or rather its recognition as a factor for civil peace and a safeguard against injustice, was largely the work of philosophers, from Pierre Bayle (a refugee in Holland) to Diderot, d'Alembert, d'Holbach, Jean-Jacques Rousseau and especially Voltaire, who was its real champion.
THE CALAS AFFAIR
From his earliest days as a playwright, Voltaire (1694-1778) indirectly attacked the scourge of fanaticism that he was to fight throughout his life. Thus in 1728, in his epic poem La Henriade, he praised King Henri IV for setting out to be the wise, enlightened monarch of all Frenchmen, whatever their beliefs. The concept of tolerance he tackled directly in his Lettres philosophiques (1734), his Dictionnaire philosophique (1766) and his Questions sur l'Encyclopedie (1772). Three. years before his death, under the title Le cri du sang innocent ("The cry of innocent blood"), he was to petition Louis XVI to review the trial of a victim of fanaticism whose cause he made famous, the Chevalier de la Barre. 
His main contribution to this war of ideas, however, is still his Traite sur la tolerance (1763). The exceptional importance of this text, apart from the cogency of its arguments, lies in the fact that Voltaire's treatise, unlike John Locke's Letter on Toleration (1690), to which he pays tribute, is not simply a philosophical dissertation. Thought here stemmed from action, from what we would nowadays describe as the writer's "commitment". For over a year Voltaire had been waging an unremitting campaign to secure the rehabilitation of a fabric merchant from Toulouse, Jean Calas, a Protestant falsely accused of murdering his son. Calas was sentenced to death, and in 1762 was broken on the wheel after refusing to confess under torture. Actually a majority of the judges were out to gratify an ignorant fanatical crowd, which accused Jean Calas, without a shred of evidence, simply because on the basis of hearsay it ascribed to Protestants a duty to kill their children if they intended to convert to Catholicism. This was supposedly the situation of the unfortunate son, Marc-Antoine Calas: in actual fact he committed suicide. …