How did one execution in Toulouse turn Voltaire into the champion of tolerance?

Because a case reached him that he came to see as an extreme injustice. In 1762 a Protestant merchant named Jean Calas was tortured and executed in Toulouse for murdering his son. Voltaire investigated, then campaigned in private and in public to clear Calas's name, and in doing so he put Catholic prejudice and fanaticism on display. That campaign, and the Treatise on Tolerance it produced, made him the age's champion of tolerance.

Calas lived in an officially Catholic society in which Catholicism was the state religion, with no legal right for individuals to practise other faiths. In October 1761 his son Marc-Antoine was found dead in the family home, and rumours claimed the father had killed him to stop him converting to Catholicism. The family said the son had died by suicide, which was treated as a heinous crime against God, and explained that they had first made it look like murder. The parlement of Toulouse still held that Calas was a murderer. He died on the wheel in March 1762, still declaring his innocence. By one account, an angry mob and zealous local magistrates drove the outcome.

Voltaire was contacted about the case and at first suspected Calas of anti-Catholic fanaticism. His investigations dispelled that suspicion, and he began a campaign. In 1764 Louis XV had the sentence annulled, and in 1765 Calas was posthumously exonerated on a procedural defect, with the family compensated. Voltaire put the affair at the centre of his Treatise on Tolerance, which was banned yet became extremely popular across Europe.