Religious distress 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 illusion about its condition is the demand to give up a condition which needs illusions. - Karl Marx
One of the reasons that the quote is probably so often misunderstood is that the full passage is often not quoted - the boldface in the above is my own, showing what is usually quoted. The italics are in the original.
As we can read, religion is meant to create illusory fantasies for the poor. Economic realities prevent them from finding true happiness in this life, so religion tells them that this is OK because they will find that true happiness in the next life. But it should be noted that Marx is not entirely without sympathy: people are in distress and religion does provide solace, just as people who are physically injured receive relief from opiate-based drugs. The quote is not, then, quite as negative as most people would portray.
In some ways, the quote is portrayed a bit dishonestly because saying "Religion is the sigh of the oppressed creature..." deliberately leaves out the additonal statement that it is also the "heart of a heartless world." It is a critique of society that it has become heartless, and a partial validation of religion that it tries to become its heart.
For Marx, the problem lies in the obvious fact that an opiate drug fails to fix a physical injury - it merely helps you forget your pain and suffering. This may be fine up to a point, but only as long as you are also actively trying to solve the underlying problems causing the pain in the first place. Similarly, religion also does not fix the underlying causes of people's pain and suffering - instead, it helps them forget why they are suffering and get them to look forward to an imaginary future when the pain will cease instead of working to change circumstances now.
Even worse, this "drug" of religion is being administered by the same oppressors who are ultimately responsible for the pain and suffering in the first place. Religion is an expression of more fundamental unhappiness and symptom of more fundamental and oppressive economic realities. Hopefully, though, humans would create a society in which the economic conditions causing so much pain and suffering would be eradicated and, therefore, the need for soothing drugs like religion would cease. Of course for Marx, such a turn of events isn't simply to be "hoped for" - in his view, human history was leading inevitably towards it.
