1624 Christianized Blacks Granted Freedom

In Virginia, the baptism of Blacks conferred special privileges. Since Virginia was a royal colony, the laws of England generally governed the colony. English law declared that a slave who had been "christened or baptised" became "infranchised." Thus, John Phillip, a black "christened" man, could testify in general court against a white man. Philip was considered a free man because he had been baptized twelve years before in England. - from Black Saga by Charles M. Christian. 

As you can see giving up your native African religion to practice Christianity could be a path toward freedom. Once you were baptized or christened you now had a soul; whereas before, if you were Black, you didn't. Later on, of course, becoming Christian didn't make you free. Many slaveholders used the Bible as a justification for slavery.