"Religion is opium for the people." Karl Marx
Straight from Wikipedia:
Marx was making a structural-functionalist argument about religion, and particularly about organized religion.[3][4] Marx believed that religion had certain practical functions in society that were similar to the function of opium in a sick or injured person: it reduced people's immediate suffering and provided them with pleasant illusions which gave them the strength to carry on. Marx also saw religion as harmful, as it prevents people from seeing the class structure and oppression around them, thus religion can prevent the necessary revolution.
How true.