After the normatively neutral (as far as an assessment of religion goes) caesura, "the cry of the oppressed," Marx concludes with the "opium of the masses" clause. It is the sting in the tail of sentence, a negative characterization that reverses the ironical, superficially positive thrust of the first two clauses, and reveals them to be ironical.
^^^ CB: Nice metaphor you use. Actually, Marx's last image is the halo 'round the vale of woe (tears), which fits your analogy , though this tail is ironically gentle, not a sting. Nice metaphor Marx uses !
^^^^^^
And it makes to sense to read it in any other way, even just taking the passage on its own, because there would be no rhetorical point in returning to a positive characterization after the caesura. That would just be wrong footed, and Marx very rarely is wrongfooted. The passage then does not read, + + 0 +, as you seem to suggest but ( + + ) 0 -! --the parentheses indicate irony.