The basis of irreligious criticism: Man makes religion, religion does not make man. In other words, religion is the self-consciousness and self-feeling of man who has either not yet found himself or has already lost himself again. But man is no abstract being squatting outside the world. Man is the world of man, the state, society. This state, this society, produce religion, a reversed world-consciousness, because they are a reversed world.
Religion is the general theory of that world, its encyclopaedic compendium, its logic in a popular form, its spiritualistic point d’honneur, its enthusiasm, its moral sanction, its solemn completion, its universal ground for consolation and justification. It is the fantastic realization of the human essence because the human essence has not true reality. The struggle against religion is therefore mediately the fight against the other world, of which which religion is the spiritual aroma. . . Religion is the sigh of the oppressed creature, the heart of a heartless world, just as it is the spirit of a spiritless situation. It is the opium of the people.
Karl Marx, Contribution to the Critique of Hegel’s Philosophy of Right,” in The Portable Atheist, ed. Christopher Hitchens (Da Capo: 2007) pp. 64-65