"Religion is the opiate of the people."
When I was much younger (a teenager) and had the typical myopic Western view that religion=Christianity, I struggled with this for a long time. From what I knew of Marx and what I knew of Christianity, I could not understand Marx's quasi-hatred of religion. For me, what Christ said and what Marx said were much easier to reconcile than what Christ said and what Western (particularly Murican) capitalism said.
What I came up with then, and as of yet I have no better explanation, is that Christians are good to their fellow man in order to be rewarded after death. Even if one follows what Christ said, the motivation for being a good member of society remains self-interest. This is appauling to the true communist. I realize now that this is not the sole reason for the above famous observation, but I remain convinced that it is morally reprehensible to be a good member of society only because of some promised future gain for one's self.
As usual, you are dead-on about the corruption of the idea of a communist state in the CCCP. I've often wondered how things might have worked out had Trotsky, instead of Stalin, ascended to power.