In "The Principles of Communism" Engles writes:
What will be its attitude to existing religions?
All religions so far have been the expression of historical stages of development of individual peoples or groups of peoples. But communism is the stage of historical development which makes all existing religions superfluous and brings about their disappearance.
The mirrors my perspective above in that if we focus on class struggle in order to bring about communism, religion will take a new form in either shifting to focus on simply the "spirituality" aspect or disappear entirely.