The Young Hegelians, including, at that time, Marx, had a specific project in the "critique" of "religion," meaning primarily Christianity. Hegel regarded Christianity as an aspect of Absolute Knowledge done up in picture form for the unwashed, along with Art and Philosophy (his philosophy, the conceptual, most correct, expression of Absolute Knowledge). In his apologetic, quietistic, keep-my-job moments, Hegel treats the then-existing Prussian state with its established Church as the End of History and realization of freedom. (Sound familiar?)
Scholars still debate about whether the old radical meant that stuff or just said because he felt he had to. The Right Hegelians, who did mean it, vulgarized Hegelian doctrine into crude apologetics for the status quo, buttressed by Christianity. Strauss, Feuerbach, and other Left (aka "Young" Hegelians) made the Prussian established church the initial target of their attack on the status quo.
Marx's comments on religion have to be seen in this context. Almost all of them are from the 1840s, most of which from the early '40s before '45) when he was an active member of the Young Hegelian movement and not yet a "Marxist." (Well, he was never a Marxist, never called himself that, and Engels later several times reported in letters that Marx said he wasn't a Marxist, but I mean, didn't yet hold the views he started to develop in The German Ideology (1845), the Manifest (1848), usw.) This includes the famous statements about "the opium of the masses."
Later on Marc would just lump religion into "ideology," but didn't articulate as as complex a theory of religion as he did in, for example, On The Jewish Question. This is less about Judaism -- although Marx, whose father was a political convert to Christianity and whose mother was still a religious Jew, had accounts to settle with Judaism -- than about what Young Hegelians should say about religious freedom and "human rights," to among other things, practice the minority religion in a state with an established church and official antisemitic policies.
Anyway, Brian, understood in context, Marx's own comments about religion are not even about the Abrahamic tradition but about mid 19th century Prussian Christianity, maybe be extension European Christianity. As far as I know he did not make any special study of nonWestern traditions or, or that matter of early Judaism, Christianity, or of Islam.
Hegel _did_ make such a study (see his three volume Philosophy of Religion), and Marx knew his Hegel, so whatever he may have knwon about, for example Buddhism was probably second hand through Hegel. Hegel's comments on nonWestern traditions are regarded by many scholars -- I am not competent to evaluate these comments, knowing little as I do about these traditions myself -- as often insightful but loaded with conscious and unconscious Orientalism and European chauvinism and even racism, as well as being limited by the state of scholarship at the time as well as by the limitations of then-available translations, since Hegel didn't know Arabic. Farsi, Sanskrit or any other Indian language, or Chinese.
However Marx doesn't seem to have been interested in thinking about comparative religion. I think there are a few swipes in later writings about idealism (antimaterialism) and purported political quietism in nonWestern religion, but as far as I know Marx just knew about that what he'd read in Hegel or other secondary sources.
____________________________________________________________________________________ Don't get soaked. Take a quick peak at the forecast with the Yahoo! Search weather shortcut. http://tools.search.yahoo.com/shortcuts/#loc_weather