After slavery deprived them of all their native traditions, the white man's religion was all that remained, and they made it their own.
The customs of christianity that have been embraced and sustained by African Americans are the ones that unite their communities and provide comfort to them in their world. The religion practiced in traditional Black churches is astonishingly different than what you see in traditional white churches.
I just posted a bit more about this which may entertain you.
[agnostic.com]
Many very intelligent people accepted Christianity as the truth. I suppose it gives answers as to how we got here, a code to live by, and hope for the future. Of course their answers are inaccurate, but it didn't matter to many of them, that's why they are big on faith.
Initially it was the only time off and social interaction they got. They mixed their native religions and culture into it to make it their own. It gave them a sense of hope and community.