Did Karl Marx Really Say That 'Religion Is The Opiate Of The People'?

Strictly speaking, Marx did not say that religion is the opiate of the people.  He said that religion is the opium of the people.  It may seem a small point, but opiates is a class of drugs, whilst opium was a popular drug in the 19th Century.
He ventured this opinion in his work 'A Contribution to the Critique of Hegel's Philosophy of Right' which was published in 1844.  Whilst it contains many ideas and concepts worth thinking about, it is generally remembered for the idea that religion kept people in harsh, oppressive conditions, with them unwilling to rise up and protest, because they felt that they would have a 'better life in heaven'.
In other words, religion dulls their pain but robs them of the will to effect change.  Marx was not criticising 'God' by saying this (as is often supposed) but rather he was criticising the organised Churches who were part of the establishment.