It's worth pointing out that one of Christianity's early selling points was that even slaves can achieve the Kingdom of Heaven and their degraded status in life would leave them with the possibility of glory in Heaven. This was shockingly progressive, but also a twisted, reactionary idea in other ways, as it encouraged slaves to bear their miserable condition patiently and wait for the world to come rather than try to improve their lot here and now by force.
African slaves in this country were taught to believe the same.
I don't think African slaves in the US were taught anything*. Whatever Christianity they adopted was through cultural diffusion. A great deal of it was mixed up with raw superstition at that.
Depends on the situation, not every slave was treated the same on every plantation. Some were taught religion, some not, some were taught to read and write (even though it was illegal). All depended on the master, state, community.