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Now, as a Swede who's never set foot on the North American continent I may be talking out of my arse, but as far as I know both statistics (a few seconds on Google gave me this) and the common stereotype say that African-Americans tend to be more religious than the (US) American population in general.
What would be required for the opposite to be true, a US where black people tend to be more atheistic/agnostic/apatheistic than other ethnic groups? (I put this in After 1900 because I would prefer a post-1900 POD, but earlier ones are acceptable too.)