'For Germany, the criticism of religion has been essentially completed, and the criticism of religion is the prerequisite of all criticism.
The profane existence of error is compromised as soon as its heavenly oratio pro aris et focis [“speech for the altars and hearths,” i.e., for God and country] has been refuted. Man, who has found only the reflection of himself in the fantastic reality of heaven, where he sought a superman, will no longer feel disposed to find the mere appearance of himself, the non-man [Unmensch], where he seeks and must seek his true reality.
The foundation of irreligious criticism is: Man makes religion, religion does not make man. Religion is, indeed, the self-consciousness and self-esteem of man who has either not yet won through to himself, or has already lost himself again. But man is no abstract being squatting outside the world. Man is the world of man – state, society. This state and this society produce religion, which is an inverted consciousness of the world, because they are an inverted world. Religion is the general theory of this world, its encyclopaedic compendium, its logic in popular form, its spiritual point d’honneur, its enthusiasm, its moral sanction, its solemn complement, and its universal basis of consolation and justification. It is the fantastic realization of the human essence since the human essence has not acquired any true reality. The struggle against religion is, therefore, indirectly the struggle against that world whose spiritual aroma is religion.
Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people.
The abolition of religion as the illusory happiness of the people is the demand for their real happiness. To call on them to give up their illusions about their condition is to call on them to give up a condition that requires illusions. The criticism of religion is, therefore, in embryo, the criticism of that vale of tears of which religion is the halo'
-Karl Marx, Introduction to A Contribution to the Critique of Hegel’s Philosophy of Right, 1844
Marx shows that religion is grounded in the inhuman social relations that humanity finds itself in. It cannot be done away with without doing away with its source. Philosophy and political economy are likewise both forms of religion that serve the purpose of trying to console one with the contradictions inherent in the material world.
'The religious reflections of the real world can, in any case, vanish only when the practical relations of everyday life between man and man, and man and nature, generally present themselves to him in a transparent and rational form.'
Karl Marx, Capital Volume I, 1867