Although Voltaire was not an atheist, he was highly critical of the rampant obscurantism and superstition associated in his day with the Catholic Church. In common with many other thinkers of the Enlightenment, he believed that the Church cynically obscured or withheld the truth in order to keep people in ignorance—all better to control them.

That explained why the Church was often so hostile to new innovations in philosophy and natural science such as the groundbreaking work of Galileo. It wanted to maintain control over the minds of the people and perceived new ways of thinking as a clear and present threat to that control. Such an attitude was not just wrong, thought Voltaire, it had potentially dangerous, life-threatening consequences.

A classic example of this was provided by the case of Jean Calas, in which Voltaire became passionately involved. Calas was a French Protestant executed on a trumped-up charge of murdering his son. To Voltaire and many others, it became obvious that there wasn't a shred of credible evidence against Calas and that he was convicted and executed solely because he was a Protestant, a member of a persecuted minority.

Calas's son had converted to Catholicism, which the state adduced as the prime motivation for his murder. Although Voltaire's involvement in the case was too late to save Calas's life, he did manage to secure a posthumous exoneration. In doing so, he revealed the truth that had been deliberately ignored by the authorities: that Calas's son had killed himself over gambling debts.

Personally, Voltaire adhered for most of his adult life to a rationalist religious worldview that would be best described as deistic. He did not believe in many of the core tenets of Catholicism, the religion of his birth, and indeed he was deeply suspicious of what he considered its irrational elements, such as miracles. However, what he most bitterly rejected was the dogmatic nature of the Church, which, in his view, placed artificial limits on human intellectual pursuits. Perhaps his most famous saying, "Ecrassez l'infame!" which means "crush the infamous thing" was in reference to the Catholic Church. By this he did not mean that the Church itself should be destroyed, but rather that theologians and sectarians argued for things that were contrary to reason, and, most importantly, promoted intolerance. It should be noted, also, that Voltaire was not simply anti-Catholic. He viewed many Calvinist sects to be as vicious and bigoted as were Catholics. The religions he portrays in a positive light in his works, such as the Anabaptist in Candide, believed in religious tolerance.