François-Marie Arouet (21 November 1694 – 30 May 1778), better known by the pen name Voltaire, was a brilliant and prolific French Enlightenment writer, historian and philosopher, famous for his wit and for his advocacy of civil liberties, including freedom of belief. His polemical novel “Candide” is critical of the passivity and acceptance that induced the philosophy of optimism, that this is the best of possible worlds (see: http://en.wikipedia.org/wiki/Voltaire ).
In Chapter 25 of Voltaire's novel "Candide", entitled "Visit to Lord Pococurante, Venetian nobleman": "Martin noticed some shelves laden with English books. "I trust", he said, "that a republican must take pleasure in the majority of those books written with so much freedom". "Yes", replied Pococurante, "it's a fine thing to write what one thinks; it's the privilege of man. In all Italy people write only what they don't think; those who inhabit the native land of the Caesars and the Antonines don't care to have an idea without permission of a Dominican. I would be happy with the freedom that inspires the English geniuses if party feeling and party spirit didn't corrupt everything estimable in that precious freedom."
Voltaire (François-Marie Arouet) (1765): “Once your faith, sir, persuades you to believe what your intelligence declares to be absurd, beware lest you likewise sacrifice your reason in the conduct of your life. In days gone by, there were people who said to us: "You believe in incomprehensible, contradictory and impossible things because we have commanded you to; now then, commit unjust acts because we likewise order you to do so." Nothing could be more convincing. Certainly anyone who has the power to make you believe absurdities has the power to make you commit injustices. If you do not use the intelligence with which God endowed your mind to resist believing impossibilities, you will not be able to use the sense of injustice which God planted in your heart to resist a command to do evil. Once a single faculty of your soul has been tyrannized, all the other faculties will submit to the same fate. This has been the cause of all the religious crimes that have flooded the earth. (Translation from Norman Lewis Torrey: Les Philosophes. The Philosophers of the Enlightenment and Modern Democracy. Capricorn Books, 1961, pp. 277-8)
Questions sur les miracles (1765)
Widely used paraphrase: "Those who can make you believe absurdities, can make you commit atrocities" (“Voltaire”, Wikiquote, https://en.wikiquote.org/wiki/Voltaire ).