Karl Marx in the introduction to his "Critique of Hegel's Philosophy of Right said:
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.
Criticism has plucked the imaginary flowers on the chain not in order that man shall continue to bear that chain without fantasy or consolation, but so that he shall throw off the chain and pluck the living flower. The criticism of religion disillusions man, so that he will think, act, and fashion his reality like a man who has discarded his illusions and regained his senses, so that he will move around himself as his own true Sun. Religion is only the illusory Sun which revolves around man as long as he does not revolve around himself.
It is, therefore, the task of history, once the other-world of truth has vanished, to establish the truth of this world. It is the immediate task of philosophy, which is in the service of history, to unmask self-estrangement in its unholy forms once the holy form of human self-estrangement has been unmasked. Thus, the criticism of Heaven turns into the criticism of Earth, the criticism of religion into the criticism of law, and the criticism of theology into the criticism of politics.
