We’ve had threads in the past about why so many African-Americans are/were Christians, even though the Christian religion might be supposed to be associated with their oppressors:
Why do so many African-Americans embrace Christianity?
How do black Christians justify being christians?
It’s more that they didn’t have a lot of choices.
Yeah, overseer’d whip 'em for failing to pluralize all the live-long day.
That’s nothing compared to the ones caught splitting an infinitive!
Cute. 
Of course I meant that it was less a case of “Believe as I believe instead of any of those other major religions”, and more a case of “Believe as I believe because that’s the only religion we’ve got in these parts.”
And not just the slaves. You wouldn’t get very far in the South (or much of the US) if you were a Christian who converted to another religion regardless of your social class. It simply wasn’t done.
Even my own grandmother was disowned by her family when she :eek: married a Catholic.
I’m having a hell of a time trying to find out if there were any slaves that belonged to the Roman Catholic Church-all I can find are references to how the RCs treated slaves in the U.S. through the years.
Too late to edit: Found book that might have the answers-Catholic Slaves And Slaveholders In Kentucky
It happened not just in 19th century America. In the earliest centuries of Christianity, the religion spread most readily through the underclass and slave populations. And that’s including times when it was frowned upon or illegal to be a Christian. (Of course slavery in Rome and Palestine was a somewhat different animal than slavery in America).
(omitted)
Which suggests a loophole that apparently didn’t work. Plantation owner buys a slave who has a pagan god tattoo.
“But Master, I just converted to Christianity.”
“Oh, well, then, you’re free!”
It is very important to never split an infinitive.
In those days, spirits were brave. Men boldly split infinitives that no man had split before.
The post emancipation abandonment of the master’s religion expected by the OP did happen to a small degree. Native blacks have certainly embraced Islam more than native whites, for example. Rastafarianism is another example.
It even makes less sense when you realize it was the deliberate efforts of a white Christian organization that caused the Christianization of slaves in the United States.
To some extent, this is still true in some areas of the South. There is little, if any, actual violence or outright persecution, but if a white guy went to a small town in southern Virginia today and went around dressed like a Buddhist monk, went around chanting Islamic prayers in Arabic, tried to hand out Scientology flyers, etc., the social situation would likely be rather chill. NYC, not so much.
They don’t always. In the Roman Empire, Christianity won a lot of slaves (with pagan masters) as converts; the religion appealed to them because it told them all souls are equally precious to God and all the saved will be equal in Heaven, and because, unlike some contemporary cults, the Christian Church welcomed anybody and joining up was free. In the long run some of those slaves probably converted their masters, or at least their masters’ children, raised as they were by slave-nurses and slave-tutors.
That has very little to do with any direct opposition by former slaves to the religion of their former owners. Those religious transformations came much later in our history.
It is pretty obvious that the OP is referring to slaves in the United States.
Most likely, they became Christians because (1) their masters pushed it hard and forbade any other mode of worship, so Islam and paganism were gradually forgotten, and (2) Christianity offered them a hopeful Old-Testament narrative about God liberating an enslaved people (a very popular subject of Negro spirituals – “Go Down Moses,” etc.).
Yes on the former-see posts #17 and #19, and not so much on latter. I don’t think the white Christian movement that went around promoting Christianity to the slaves were doing so to convince them that they would soon be liberated.