Marx meant that religion provides an outlet for the material problems of the oppressed and thus runs counter to revolutionary action at its core. We know this doesn't mean religious people can't be revolutionary, but he did make a good point about it having the capacity to distract or to act as a substitute for demanding material change.

I recently heard a local Muslim religious leader give a speech about Palestine. This particular person decided to focus on "the" meaning of hijab (it of course means different things in different contexts) and to then say that as Muslims, it must be understood that this unbearable suffering in Palestine is only temporary, that it can be accepted because the afterlife will vindicate them.

On one hand, I would not criticize anyone for using religion as a way to heal from oppression. At the same time, you can see how "only the next life matters" could lead someone to place less emphasis on materially fighting against oppression and therefore do a worse job of opposing it.

We of course see this kind of thing in all religions, they provide explanations for injustice that usually place blame outside of what can be materially helped and justice that is entirely outside the scope of our lives. Many have the same basic idea that this life is temporary and doesn't matter in comparison to the infinite later (whatever that might be).

When it comes to organized religion, one can also see how it becomes merged with the ruling class, becoming an inherent component of social order and control and adapting its beliefs to what is needed by the ruling class. For example, early Christianity was pretty clear about all interest on debts counting as usury. There was no distinction in language whatsoever. But the Roman ruling class depended on debt, so the distinction was invented and suddenly there were good moneylenders and bad moneylenders and the ruling class's status quo was - you guessed it - good moneylenders!

Anyways, I'm getting off-track. Marx was talking about religion offering a false consciousness that detracts from revolutionary behavior.

I do think I should probably mention something else, though: many communist movements attempted to impose atheism and/or very strict secularism on populations that were not receptive to it and thereby shot themselves in the foot. It's good to understand Marx's point about what religion can offer and do but to not take it to a false extreme that deposing religion is the first priority. You'll end up, ironically, actually feeding organized religion that will redirect revolution away from being proletarian, as the people you should be able to recruit and radicalize will instead go to the alternatives, including religious organizations.