Basically, Marx saw religion solely as a means whereby the ruling classes suppress ordinary people. Marx was correct that rulers use religion to discourage rebellion but this was clearly very much simplified. Egalitarian tribal hunter gatherer societies usually have religious or spiritual beliefs, hierarchical societies are not alone in this. Further those who want to fight the rulers in their society are likely to develop interpretations of religion that suggest their fight is virtuous, Liberation theology is an example.
Karl Marx's most famous saying in its context, with emphasis added:
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 encyclopedic 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.
- Contribution to Critique of Hegel's Philosophy of Right, 1843
- Deutsch-Französische Jahrbücher (journal by Karl Marx and Arnold Ruge), 1844
This article was originally at the Beacon Library (now defunct).