Marx isn't anti-religion because "lol man in the sky silly". Marx is anti-religion because it is the product of an inverted world, a class society. Marx believes that, if class society is abolished, if the topsy-turvy (this is the word he uses in e.g. "On 'The Jewish Question'" or the rest of "Contribution to the Critique of Hegel's Philosophy of Law") world is turned upright, religion will wither away into nothing. Marx believes this is only possible when all the social relations that underlie society are laid bare in an understandable form (i.e. with communism), and that it will not disappear until the topsy-turvy material world disappears.

As Marx states in the former essay, Marx doesn't believe religion can be abolished by decree (and he points out that state secularism is often just reskinned Christianity). In both essays, Marx actually details how the state itself, money itself, are religions; for Marx religion is the work of human mind alienated from humans and dominating them.

In the full quote (which emizeko posted) Marx outright refers to religion as the general theory of this world (i.e. the topsy-turvy world), as an encyclopaedic compendium. Elsewhere (either in Contribution to the Critique or in On the Jewish Question) Marx refers to religion as a register of the theoretical struggles of mankind. Based on Marx's usage of various bible quotes and themes in his work (even Capital), it seems likely that he treated religion as he would e.g. liberal economists or members of parliament (i.e. with critical analysis and an eye towards useful stuff for his own critique)