You no doubt have heard this quote but I was re-reading Marx's critique of Hegel's 'Philosophy of Right" and can't help but marvel at Marx's command of language. Most people butcher this quote, and only include the last part. In context, it is far more powerful.
"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, Critique of Hegel’s Philosophy of Right