According to Karl Marx, religion is like other social institutions in that it is dependent upon the material and economic realities in a given society. It has no independent history; instead it is the creature of productive forces. As Marx wrote, “The religious world is but the reflex of the real world.”
Religion can only be understood in relation to other social systems and the economic structures of society. In fact, religion is only dependent upon economics, nothing else — so much so that the actual religious doctrines are almost irrelevant. This is a functionalist interpretation of religion: understanding religion is dependent upon what social purpose religion itself serves, not the content of its beliefs.
Marx’s opinion is that religion is an illusion that provides reasons and excuses to keep society functioning just as it is. Much as capitalism takes our productive labor and alienates us from its value, religion takes our highest ideals and aspirations and alienates us from them, projecting them onto an alien and unknowable being called a god.
Marx has three reasons for disliking religion. First, it is irrational — religion is a delusion and a worship of appearances that avoids recognizing underlying reality. Second, religion negates all that is dignified in a human being by rendering them servile and more amenable to accepting the status quo. In the preface to his doctoral dissertation, Marx adopted as his motto the words of the Greek hero Prometheus who defied the gods to bring fire to humanity: “I hate all gods,” with addition that they “do not recognize man’s self-consciousness as the highest divinity.”
Third, religion is hypocritical. Although it might profess valuable principles, it sides with the oppressors. Jesus advocated helping the poor, but the Christian church merged with the oppressive Roman state, taking part in the enslavement of people for centuries. In the Middle Ages the Catholic Church preached about heaven, but acquired as much property and power as possible.
Martin Luther preached the ability of each individual to interpret the Bible, but sided with aristocratic rulers and against peasants who fought against economic and social oppression. According to Marx, this new form of Christianity, Protestantism, was a production of new economic forces as early capitalism developed. New economic realities required a new religious superstructure by which it could be justified and defended.