Have the people changed?
"Opiate" had a slightly different set of connotations in Marx's time -- the drug was hallucinatory, potentially destructive, but also medicinal. That's important to remember when considering the quote: Marx is saying that religion is both a diversion and a sort of salve. The point is not quite that religion dumbs the proletariat down -- it's that it genuinely soothes and pacifies the people, at least enough to prevent them from revolting against worldly oppression. Which is a problem for Marx.
If you're understanding "opiate" in the modern sense, you have a lot of easy, kneejerk, unsatisfactory answers: credit or entertainment or consumer culture. The truth is that all of those things both soothe and divert to various degrees, but none quite like religion. Religion is still the opiate of many people, and I mean that in a more positive sense than Marx; he was mistaken to call for its overthrow. Selfish or overly literalistic interpretations of religion (any religion) can turn us obsessively inward or fiercely outward. It can give us an ugly sense of purpose and self-satisfaction, a set of guidelines -- unexamined -- to impose on our neighbors and universe. In this sense it is a destructive addiction.
But at its best (and, I believe, far more often) some form of faith offers a sense of continuity with the world -- the worlds dead and living, the worlds to come. The comprehensible is also eternally mutable: our countries may crumble, our politics fall out of favor, our revolution disappoint. There can be no final uprising, no perfect and permanent commune a la Marx. In the face of uncertainty, we may form a relationship with mystery, which does abide. We may acknowledge a baseline continuity between ourselves and all others, and choose to behave accordingly. That is the spine of faith, which runs counter to neither science nor common sense. And I think it can serve as the philosophical foundation for equitable relations between human beings, rather than the impediment.