Not only Religion is a drug used by Politickers (History of Karl Marx describing Christian Theology as a drug is concerning Christianity when it was not splitted from Political Power, i.e. until 1800 in France. In true Marxist History, Christian religion died in Europe around 1800 and was replaced by nazism of G.W.F. Hegel; Marx is describing German Philosophy as the new Religion, stronger than the former one because the members of this new Cathedral are denying making a Religion. And in fact it is not true to think that French people are less religious than USA people for instance; most of French teachers of the more than three million French Education system employees are acting as real clerks.)
One can even say that the biggest the State is, the more centralized: the strongest must the Religion be. Lenin for instance guessed that Communism or Marxism would turn in big Soviet Union in a Religion 'as Christianity turned in a Religion in France in XVIIth Century', due to strong central power (Lenin does understand Christianity better than today Pope Benedictus XVIth, full of crazy German Philosophy!).
Not only Religion is a drug: I mean Drug is a religion too. Sole use of drug by someone is proving he is under religion. Effects are similar. Due to his link with Greek Science, Marx does think that religion is the way not to know who you are, a veil between the Sky and Human people. This schizophrenia or melancolia is just faster with drugs but desire of drug is the same than desire of religion: abandonment to a power stronger than you are.
It was probably a kind of joke in young Marx mind to attack 'existentialism', which is the religious background of nazism (see Heidegger, member of NSDAP), to fight against it as this was coming from the Devil. But now we can see that existentialism is from Satan and Marx was true.
In fact Religion as we can see in some Pagan civilizations such as Roman Civilization can be 'under the Devil', i.e. not make the difference between God and Satan. It is due to a bad level of Scientific knowledges. You can find it in Greek civilization and this is Pythagorean satanic religion strongly condemned by Aristotle and Francis Bacon later.