There are a few different ways to approach this question. One is why slaveholders would bother converting their slaves to Christianity. For a solid amount of time that the institution of slavery existed in the US (with definite regional variance), slaves weren't actually really allowed to learn about Christianity—especially in the areas most reliant on slavery. The reasons for teaching slaves about their masters’ religion ranged from the belief that slaves didn't have souls to the fear of inspiring an uprising to simple indifference. It was in and around the 17th century that slaves in the North started to learn about Christianity; the South didn’t see this until the mid-18th century into the 19th century^1 (some reference the various waves of Great Awakenings, but I think that categorization oversimplifies things). It’s been argued that the societal conditions of 17th- and 18th-century New England—which consisted of a “fairly rigorous variant of Protestantism”^2 —made it so that slave owners saw the conversion of their slaves as something of a Christian duty. Another argument that tries to explain why slave owners might want to convert their slaves is that instilling Christian values and teachings—particularly focusing on the aspects which encourage obedience—would make slaves more docile, but this reasoning seems to be based more on intuition, bias, or expectation rather than what the historical records show.
That brings us to a second approach to your question: why would slaves want to convert to the religion of their masters? The practicality argument can definitely be made: by allowing themselves to be taught the Christian faith, slaves gained several valuable tools—literacy and an expanded English vocabulary in particular—that they would have had a very difficult time acquiring otherwise.^3 Somewhat along those lines, Jon Butler makes the case that by effectively erasing African spiritual traditions, the institution of slavery in the US also erased the ways in which the enslaved African peoples understood and perceived life. According to Butler, this basically created a vacuum which Christianity was all too happy to eventually fill.^4 In more of a theological sense, Christian faith also provided slaves hope and thus the will to survive the tragedy that was American slavery. Stories from the Bible, especially the Exodus of the Jews from Egypt, promised God’s deliverance from slavery.^5 There are some claims that this deliverance was only thought to be spiritual (i.e., going to heaven), but I think there’s enough evidence to assert that many slaves believed that God would play an active role in freeing them.
A third approach is to ask exactly how effectively slaves in the US were actually converted to Christianity. Daniel Fountain found, after some fancy quantitative analysis, that about 40 percent of antebellum slaves claimed to be Christian^6 while the best estimate (at least that I’m aware of) for the general population at the time is 37 percent.^7 This indicates that while Christianity was not ubiquitous among slaves before 1860, they were approximately in proportion with the contemporary general population. In addition to looking at the straight numbers of converted slaves, it’s also important to look into the sincerity by which Christian beliefs were actually held—were slaves just trying to take advantage of an existing institution or were they genuine in their subscription to Christianity? Of course, the answer is not so dichotomous; there are many examples of both in the form of historical accounts, several of which are absolutely tragic. One man named Toney was in such [understandable] despair about his brutal enslavement that he wanted to kill himself. The scripture he had taken to heart, however, made it clear that suicide was not an option in the eyes of God. Instead of killing himself, he killed the 5-year-old daughter of his master (if I understand correctly, he saw the girl as more “pure” and it was therefore less sinful to kill her as she would be more likely to go to heaven, but I’m not completely sure about that) and then immediately ran into town to turn himself in so that he would be executed.^8 This account certainly demonstrates an intense degree of dedication to religious faith, but that doesn’t necessarily mean the sincerity was or was not universal.
Like pretty much any question in history, the conversion of the enslaved to Christianity in the US is ridiculously complex and without any single, straightforward cause. There are volumes of books about black theology, implications of slave religiosity (both divine and mundane), and contemporary accounts from several perspectives from European third parties to slaveholders to the slave himself, but I hope my response adequately answers your question. I’m happy to elaborate on anything I might not have made clear!
Jared Hardesty, “An Angry God in the Hands of Sinners: Enslaved Africans and the Uses of Protestant Christianity in Pre-Revolutionary Boston,” Slavery & Abolition 35, no. 1 (2013): 66–83.
Richard A. Bailey, Race and Redemption in Puritan New England, New York, Oxford University Press, 2011, 9.
Hardesty, 66.
Jon Butler, Awash in a Sea of Faith: Christianizing the American People, Cambridge, MA, Harvard University Press, 1992, 129–30.
Cheryl J. Sanders, “Religious Conversion, Ethics, and the Afro-American Slave: Evaluating Alternative Approaches,” The Journal of Religious Thought, 2001.
Daniel L. Fountain, * Slavery, Civil War, and Salvation: African American Slaves and Christianity, 1830–1870*, Louisiana State University Press, 2010.
Roger Finke and Rodney Stark, “Turning Pews into People: Estimating 19th Century Church Membership,” Journal for the Scientific Study of Religion, 25, no. 2, 1986. This estimate is based on some other fancy statistical tests that assess how many people actually attended their local churches; while it may seem low, it’s backed up by some pretty substantial evidence.
Hardesty, 78.