This is a hot topic of debate and it's difficult to know the true answer. Historian Ira Berlin in "Generations of Captivity" makes the argument that enslaved people did not really christianize until the 1790's-early 1800s when imports of enslaved people from Africa came to an end and slaveholders began to systematically evangelize enslaved people. Archaeological evidence at numerous sites like the African Burial Ground in NYC, the Jordan Plantation in Texas, Annapolis, MD, show that bakongo cosmograms, conjuring magic, and ritual offerings of food to the dead appeared to be significant aspects of African-American faith in the era of slavery. In my own work, I've encountered a two page document recounting a 'conjure man' enslaved in the appalachian salt mines who converted to christianity as late as the 1850s. Based on this evidence, I'd suggest that a sizeable minority tried to construct an African religious identity in the new world.