Religious stress is at the same time the expression of real distress and the protest against real distress. 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.
The abolition of religion as the illusory happiness of the people is required for their real happiness. The demand to give up the illusions about its condition is the demand to give up a condition that needs illusions. The criticism of religion is therefore in embryo the criticism of the vale of woe, the halo of which is religion. Criticism has plucked the imaginary flower from the chain, not so that man will wear the chain without any fantasy or consolation but so that he will shake off the chain and cull the living flower.

Karl Marx