Which statement is used in reference to religion and belongs to Karl Marx?

The opium of the people (or opium of the masses) is a dictum used in reference to religion, derived from the most frequently paraphrased statements of German sociologist and economic theorist Karl Marx: "Religion is the opium of the people." In context, the statement is part of Marx's structural-functionalist argument that religion was constructed by people to calm uncertainty over their role in the universe and in society.

The quotation originates from the introduction of Marx's work "A Contribution to the Critique of Hegel's Philosophy of Right", which he started in 1843 but which was not published until after his death.

Karl Heinrich Marx (5 May 1818 – 14 March 1883) was a German philosopher, economist, historian, sociologist, political theorist, journalist and socialist revolutionary. Born in Trier, Germany, Marx studied law and philosophy at university. Due to his political publications, Marx became stateless and lived in exile with his wife and children in London for decades, where he continued to develop his thought in collaboration with German thinker Friedrich Engels and publish his writings, researching in the British Museum Reading Room.

His best-known titles are the 1848 pamphlet "The Communist Manifesto" and the three-volume "Das Kapital" (1867–1883). Marx's political and philosophical thought had enormous influence on subsequent intellectual, economic and political history. His name has been used as an adjective, a noun, and a school of social theory.