Men never do evil so completely and cheerfully as when they do it from religious convictions.

[Jamais on ne fait le mal si pleinement et si gaiement, que quand on le fait par un faux principe de conscience.]

Blaise Pascal (1623-1662) French scientist and philosopher

Pensées, ch. 14, Appendix: Polemical Fragments #895 (1669) [tr. Trotter (1910)]
    (Source)

Also labeled as Part 2, Article 17, # 53. Sometimes also shown in slightly shorter French as "Jamais on ne fait le mal si pleinement et si gaiement que quand on le fait par conscience."

(Source (French)). Alternate translations:

• "We never do evil so fully and cheerfully as when we do it out of conscience." [#813 (#895), tr. Krailsheimer]
• "We never do evil so completely and cheerfully as when we do it out of conscience." [Miscellaneous Thoughts 7: Sellier #658/Lafuma #813, tr. Ariew]
• "Men never do evil so completely and cheerfully as when they do it conscientiously."