"Religious suffering is, at one and the same time, the expression of real suffering and a protest against real suffering. Religion is the sigh of the oppressed creature, the heart of a heartless world, and the soul of soulless conditions. It is the opium of the people."

My interpretation is a bit different. Rather than suggest that taking away religion will facilitate revolution, it seems to me that Marx is arguing that revolution will obviate the need for religion (being a symptom rather than the disease). Until the source of people's suffering--which is to say an indecent and inhumane economic system--is abolished, religion would (should?) remain intact as a source of solace as a response to said suffering. Opiates are both painkillers, not necessarily sedatives.