The passage you have in mind runs thus:
Religious suffering is, at one and the same time,
the expression of real suffering and a protest
against real suffering. Religion is the sigh of
the oppressed creature, the heart of a heartless
world, and the soul of soulless conditions.
It is the opium of the people.
The abolition of religion as the illusory happiness
of the people is the demand for their real happiness.
To call on them to give up their illusions about their
condition is to call on them to give up a condition
that requires illusions. The criticism of religion is,
therefore, in embryo, the criticism of that vale of tears
of which religion is the halo.
Criticism has plucked the imaginary flowers
on the chain not in order that man shall continue
to bear that chain without fantasy or consolation,
but so that he shall throw off the chain and pluck
the living flower. The criticism of religion disillusions
man, so that he will think, act, and fashion his reality
like a man who has discarded his illusions and
regained his senses, so that he will move around
himself as his own true Sun. Religion is only
the illusory Sun which revolves around man
as long as he does not revolve around himself.
(Karl Marx, "Introduction to A Contribution to the
Critique of Hegel's Philosophy of Right,"
Deutsch-Französische Jahrbücher, February,1844 <http://www.marxists.org/archive/marx/works/1843/critique-hpr/intro.htm>)
Marx's criticism of religion was indeed more nuanced than a notion that the idea of God is delusion, as were those of Engels and better sorts of adherents to the Marxist tradition.
Thinking religious people, clerics and lay persons, on the broadly defined Left as a matter of fact have taken up Marx's challenge: their religions sharply criticize, both in secular and religious terms, the vale of tears that this world indeed is for many, and actively try to mobilize their adherents to change this world. It's about time that irreligious leftists fully recognize that religious leftists are morally, politically, and intellectually equal to us and that they are quite often better at organizing than we are, in many parts of the world. They have also made efforts to rid their religions of racism, sexism, homophobia, and so on. That's work in progress, but the same must be said about irreligious leftists' ideas and practice in this regard, too. It is possible that irreligious leftists, while having an easier time overcoming sexism and homophobia, may have more trouble overcoming racism than religious leftists. Many religions have national and international structures through which people of different races, nations, etc. are compelled to work together, the structures that irreligious leftists generally lack.
Many irreligious leftists protest that they have no trouble working together with religious people, but, when they say that, they generally mean they have no trouble working together _with religious leftists on issue-by-issue basis_, like opposition to the Iraq War and support for higher minimum wages. Whether irreligious leftists can work with religious people who are not clearly on the Left or Right at present and have a mix of progressive and reactionary ideas, on a project of social change that goes beyond this or that single-issue movement, is an open question. -- Yoshie <http://montages.blogspot.com/> <http://mrzine.org> <http://monthlyreview.org/>