That whole bit about religion being the "opium of the masses" was a very misleading half-truth. It can be a drug that puts people to sleep ... as can any other corrupted institution. Sports is another cultural institution that can have the same effect for the same reasons. Or you could look at music or dance or literature or any other cultural institution at all, and you will see that when it is corrupted, it acts like a drug on the population.
Jews, I am sure, are keenly aware of this and I'm sure Marx was. But he gives this selected fact and lies by omission.