Cacs taught blacks Christianity so blacks wouldn't revolt. Period. Cacs used the Bible to both justify their crimes and to discourage blacks from overcoming their oppression. In this way Christianity was used as a tool of oppression.
Actually some Black people, especially from the Kingdom of the Congo, were already Christian before they ever got to America. However, your point about how Christianity was used by the slave owners is accurate.
http://www.everyculture.com/wc/Brazil-to-Congo-Republic-of/Bakongo.html