“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".
Original
Mais, monsieur, en étant persuadés par la foi, des choses qui paraissent absurdes à notre intelligence, c'est-à-dire, en croyant ce que nous ne croyons pas, gardons-nous de faire ce sacrifice de notre raison dans la conduite de la vie. Il y a eu des gens qui ont dit autrefois: Vous croyez des choses incompréhensibles, contradictoires, impossibles, parce que nous vous l’avons ordonné; faites donc des choses injustes parce que nous vous l’ordonnons. Ces gens-là raisonnaient à merveille. Certainement qui est en droit de vous rendre absurde est en droit de vous rendre injuste. Si vous n’opposez point aux ordres de croire l’impossible l’intelligence que Dieu a mise dans votre esprit, vous ne devez point opposer aux ordres de malfaire la justice que Dieu a mise dans votre coeur. Une faculté de votre âme étant une fois tyrannisée, toutes les autres facultés doivent l’être également. Et c’est là ce qui a produit tous les crimes religieux dont la terre a été inondée.
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Voltaire 167
French writer, historian, and philosopher 1694–1778Related quotes

“Those who can make you believe absurdities can make you commit atrocities.”
Preface (p. xiii; quoting Voltaire)
Irreligion: A Mathematician Explains Why the Arguments for God Just Don’t Add Up (2008)
The Art of Doing Science and Engineering: Learning to Learn (1991)
Source: Drenai series, The King Beyond the Gate, Ch. 13
Context: And now I am going to tell you a great truth, and if you are wise, you will take it to your heart. All men are stupid. They are full of fear and insecurity— it makes them weak. Always the other man seems stronger, more confident, more capable. It is a lie of the worst kind, for we lie to ourselves... How do you know? You do not! You listened to the voice of your inadequacy, and because you believed, you are in my power. If I draw my sword, you are dead!

“Never grant anyone the power not to make you believe to your dreams.”
Original: Non concedete mai a nessuno il potere di non farvi credere ai vostri sogni.
Source: prevale.net