A really good book I would recommend for understanding the concepts behind African slaves adopting Christianity (and the early origins of it) is Rebecca's Revival: Creating Black Christianity in the Atlantic World by Jon F. Sensbach. It centers around the Danish Caribbean colony of St. Thomas in the early 18th century and a freed woman of African and white descent named Rebecca Protten who converted and became a strong force for spreading Christianity among slaves - and even suffering persecution for it.
In this book, it shows Moravians (another small group/denomination of protestant Christians) sending people to the colonies to convert slaves. It appears that there were diverse reasons for slaves to take to Christianity. Some saw it as spiritually freeing, some saw it for the hope it offered, and some even saw it as a way to differentiate themselves from their masters since it allowed for criticism of the master's hypocrisies in their ways of living and treating slaves. It became a way to be defiant towards slave masters, and slave masters and their supporters began opposing the spreading of Christianity to slaves. The previously mentioned Rebecca became persecuted for it. I would highly recommend reading the book, it's a great insight into slave history, religious history, Danish colonial holdings in the Caribbean, and more.

The United States and its slaves are quite different in terms of conditions for slaves with regards to freedom of religious practice when compared to other areas of the "New World". On the sugar plantations of Haiti, and the indigo farms of Louisiana, the French were in the habit of not managing what little private time were afforded to their chattel, and had no interest in saving their souls. They would die from overwork soon enough anyway (so obviously they didn't have much time on their hands for religious education, being worked to death tends to take up a whole day), and the slave owner rarely came in contact with his slaves. In the U.S., a slave owner tended to know his slaves a little better, and the common American approach to all non-Christian peoples is to force the issue of salvation, whether it was welcomed or not. This fervor for spreading the gospel was often most heated during the successive "Great Awakenings" that would periodically occur. Ironically, it was the second Great Awakening that saw abolitionism in America pick up steam.

In the North, up until the time that Methodist and Quaker preachers started preaching abolition, blacks didn't have an interest in Christianity. It wasn't until the 19th century that the abolitionist movement in Methodism got really heated, but it started at the end of the 18th century with John Wesley. The Quakers have a less shaky stance on abolition as an organization, and actually led the movement to have slavery abolished in Britain.

It wasn't until the Second Great Awakening in the South that Baptist preachers abandoned preaching manumission and started ministering to slaves more heavily, and their message was one of the Christian Bible supporting slavery. This didn't work very well however, as Blacks after the emancipation had very different views of the Bible, ones that focused on slaves being liberated from bondage, the central story of that book being Moses leading the Israelites out of Egypt. (A story which is still popular among Christian blacks in America today)

So in the North and South you have different reasons for the adoption, but both center on the religious belief that slavery was wrong (despite the Southern Baptists attempting to use religion to pacify slaves) and that it was the slave owners that would suffer for their sins in the next life, as well as whites advocating abolition in the name of Christ.