I've always been interested in the use of religion in the Civil Rights movement as a way to unite people, but how did it come to pass that so many black slaves became Christian in the first place? Was it forced upon them by their owners? Was there a targetted effort of churches to convert? What happened with the traditional faith of the African people?
Whilst I'm a bit pressed for time to give you a new answer today (though I'll happily address any follow-up questions or requests for more detail you might have when I get a chance tomorrow), I would draw your attention to an answer that I wrote here about a month ago, in response to a very similar question on the Christianisation of slaves by the white planter class.
I'm not one of the scholars here, but your question brings to mind an interview I read for a class on the Letters of Paul. In the interview, Howard Thurman recounts his grandmother's experiences with Christianity while growing up a slave.