The roots of Christianity was planted in Kongo and Angola as far back as the 1480's, which was even before Columbus stumble bummed his ass into the New World. Most slaves were not even shipped to the Americas until between the years 1600 and 1790, so many of the people of Kongo and Angola were already Christian when they go the Americas.
African Christianity in Kongo | Essay | Heilbrunn Timeline of Art History | The Metropolitan Museum of Art
Fwiw, most slave owners during the majority of the period of slavery did not care that the slaves were not Christian. In fact many slave owners provided their Muslim slaves with Korans. Most of this conversion junk did not happen until the "Second Great Awakening", which was in the 1800's. Many abolitionists pushed it on the slaves during that period.
Religious Transformation and the Second Great Awakening [ushistory.org]
From the 1600's and into the 1800's there are multiple examples of the slave owners not caring what religions their slaves had, including Islam. Many slave owners actually provided their slaves with Korans and some of those Korans have been donated to major universities throughout this Country. The Muslims seem to be the slaves that were forcibly converted by force to Christianity by the English, Portuguese and Spanish in the Americas and that was probably due to the Muslims constantly leading slave revolts.