This is a really interesting question, one that has been discussed by historians for quite some time, so I'll tackle it from a few directions. I also want to describe what religious culture was like in enslaved/black communities before explaining why they didn't throw off their religion once they found their freedom.

18th Century

First, you're correct that through the Eighteenth Century, it became common practice to encourage or require slaves to practice Christianity. I'm going to focus on what religion looked like at George Washington's Mount Vernon since we know a considerable amount of what the religious culture was like there, and since it is a fair representation of what the cultures were like on other large plantations.

Christianity amongst Washington's slaves dates back decades before Washington became president. But Washington didn't require his slaves to convert to Christianity. In an 1784 letter, Washington wrote:

"If they are good workmen, they may be of Asia, Africa, or Europe. They may be Mahometans [Mohammedans/Muslims], Jews, or Christians of any Sect, or they may be Atheists."1

While Washington did not require his slaves to convert, we know that many of these slaves, most were second or third generation immigrants, did convert. We know that Bruton Parish Church in Williamsburg, at least 11 of George Washington's slaves that were christened in the 1760s, and this was just the start of their Christian conversion. Of the over 300 slaves who lived and worked at Washington's Mount Vernon plantation, they belonged to three denominations (Baptsists, Methodists, and Quakers), with 23 of slaves being formal members of a church by 1790. (While this may seem low to some people, I would like to point out that religion during this period was not practiced as much as one might expect. Only about 15 - 20% of the American population belonged to a church and attended at least one church service a year, so this falls in line with their contemporary culture. 1) Evidence also suggests that Mount Vernon had one enslaved spiritual leader who acted as a Christian preacher. It is quite possible there were more. However, it would be unfair to say that Christianity was the only religion at Mount Vernon at all. Islam was represented amongst at least two of Washington's slaves during his lifetime. But the truth is, Islam would have been a harder religion to follow while being enslaved. Workweeks existed from Monday - Saturday, meaning they could not respect Friday's as their day of rest, nor would they have been permitted to stop working to pray five times a day. Other cultural issues would have been difficult as well to follow, as historians at Mount Vernon stated:

On most plantations, pork formed a significant part of the rations provided for slaves, and alcohol was often utilized as a reward or was dispersed during times of especially hard work, in the belief that it was necessary for health. This certainly would have made it difficult for individuals to follow Islamic dietary guidelines. In addition, pilgrimage to Mecca would have been out of the question for anyone with slave status in the Americas. [2]

So while it was a challenge to follow the religion of Islam on plantations, it would be much easier to follow Christianity, since other slaves would have been sharing in that religious culture as well. Religion has the powerful ability to bind people together, and it appears that this bound slaves together, especially as they went through their own tribulations as slaves. What is important to note is that Christianity began to spread amongst the enslaved, and would continue to do so in the 19th century. It's likewise important to realize that this trend of slave owners not forcing Christianity upon their slaves in the 18th century is a trend found throughout America.

Certain branches of Christianity, including Quakers, Methodists, and Evangelicals welcome black Americans into their churches, "where their emotional responses helped to inspire the relatively staid whites to lose their inhinbitions and feel the Holy Spirit. A Methodist declared, "In general the dear black people, that profess Religion, are much more engaged than the whites. By 1790 the great majority of Virginia slaves had embraced Christianity Fervently."[3]

19th Century

Christianity greatly spread and expanded in America through the first half of the 19th century. In a period often referred to as the Second Great Awakening, which was a religious revival movement started in New England in the late 1790s but spread to the rest of America through the 1800s. More people found religion and began going to church regularly in waves that America had never seen. For instance, in 1775 America had about 1,800 ministers in all 13 colonies. That numbered swelled to 45,000 by 1845.[2] Certain religions, like Methodism, their membership grew at over triple the rate of America's population growth in just the first few decades of the 1800s. All of this impacted religion amongst both free blacks and enslaved populations. By the 1850s, Christian religious denominations became focal points within black culture.

By the 1850s, it became important for slave, especially enslaved women to teach the tenets of the Christian faith to their children. In a letter to John Cocke, a literate enslaved woman wrote:

We keep up with the family Prayers every morning. I does my best I can teaching the Children but I can never get more than two and sometime three little ones of week days.[4]

This letter is representative of enslaved culture through this period, where it became important for slaves to want to educate their children on Christianity (and other educational elements, like reading and writing).

Also through this period, slaves began looking at other, more hopeful elements of the Christian religion. For instance, there was a strong movement within slave culture to discussing and preaching about God's deliverance of the Isralites from their enslavement in Egypt, so to did many slaves believe God would do the same for them. One slave in particular, Harriet Tubman, was nicknamed "the Moses of her people," by the abolitionist, William Garrison. This provided hope, so much hope that enslaved people wanted and perhaps needed to hold onto. It's also important to note that enslaved people also created their own culture within Christian circles. They adapted Christian hymns to match their own growing societies and even started creating their own songs throughout the 19th century. The Christian faith, from their perspective, became "theirs" and was not simply the religion of their masters.

Conclusion

Enslaved culture adopted Christianity on their own terms very early on. While the religion was used by their masters to justify their enslavement, enslaved Americans did not see it the same way. Many converted on their own terms, and as Christianity grew and changed in the 19th century, they grew and changed with it. So when emancipation happened in the 1860s, these people had been Christians for many generations and became intertwined into their culture, so much so, that they did not feel compelled to throw off their religion.

1). Jon Butler, “Magic, Astrology, and the Early American Religious Heritage, 1600-1760” The American Historical Review, Vol. 84, No. 2 Apr. 1979. 318.

2) Nathan Hatch, The Democratization of American Christianity. Oxford University Press, 1990. 4.

3) Alan Taylor. The Internal Enemy: Slavery and the War in Virginia. W. W. Norton & Company. 2013 .36

4) Dorothy Sterling. We Are Your Sisters: Black Women in the Nineteenth Century. W.W. Norton & Company. 1997. 53

Edited: Added a source.

As someone who has studied non-Christian black religiosity since the 19th c, I'd like to add a couple of things that may help shed some light on some of the issues mentioned here, as well as on later rejections of Christianity by African Americans, especially by the children, grandchildren, and great-grandchildren of slaves.

First of all, many people today believe that slaves simply passively accepted an oppressive religion forcibly imposed by the Masters. However, while it is true that there were various waves of whites trying to convert African Americans and that, especially since the 1830s these white proselytizers emphasized the "Christian" concept of "slaves, obey your masters", many slaves were aware of what was going on and they consciously rejected White Christianity. In fact, there was a very strong belief among enslaved Christians that they were the only true Christians, and that whites were fake christians who did not even live up to the ideals they preached. We have multiple accounts of slaves making that claim, as well as several related assertions, such as the idea that the Bible used by whites was a phony (they said a true Bible may exist, but that was not what whites had), or that whites had inserted pro-slavery verses into it. Some slaves, and many of their descendants for several generations, went as far as completely rejecting the concept of a written Bible, saying instead that their Christianity was a "religion of the heart", and their true scriptures were found in spirituals (songs) and extemporaneous preaching by other slaves. So slave Christianity--and after Emancipation, the Christianity of many rural African Americans (90% of the black population)--was understood as completely unique to the African American community and a religion that did not emphasize oppression.

One of the reasons so many slaves embraced Christianity in the first place was that it was a cultural unifier for the diverse ethnic and religious groups that had been brought to America. Several elements of Christianity were recognized as similar to their African religions by slaves from a variety of regions. Prominent among these elements were a belief in a High God, baptisms and funeral rites--so embracing Christianity meant getting a publicly acceptable forum for expressing old religious yearnings. Many slaves also used Christian services to perform African religious rites that were widespread in Africa but had been performed for different gods and ancestors depending on the ethnicity or home region of the slave--Christianity served as a publicly acceptable means to find a common ideological/religious ground to bring these different people's practices together. The most obvious example of this is the ring shout. Many African American Christians also believed in Hoodoo, folk magic, which was a mix of old African magical practices and European esotericism, and frequently Christian elements were intermixed as well.

Keep in mind that prior to being brought to America, Africans did not generally look at the world in terms of race. The emphasis was on ethnicity or tribe generally. But in America, race was the defining social factor, so when their Christian identity emerged, race played a prominent role in it--as did the notion that "black" people (as a race) were a special holy people, whose trials and tribulations in America could easily be identified with biblical figures, especially the Israelites, unlike white people, who (from the slaves' perspectives) had their clearest biblical counterparts with the Pharaohs and Romans/gentiles. Some currents in Black American Christianity therefore contained ideas about a coming Judgment Day when whites would be destroyed, just as Joshua destroyed the Canaanites. Jesus was also frequently conflated with Moses and images from Revelation, and depicted as a redeemer of the oppressed. This was very different from the messages preached in white churches.

As for the overt rejection of Christianity by former Christians--the religious borders aren't as clear as they seem at first glance. In the 1880s you get the rise of black Jews, but these people came out of the Holiness movement (a Christian sect that had come out of Methodism), and basically the only thing they did different was, instead of saying they were "similar" to the Israelites of the Bible, they said they were the Israelites' "literal" descendants, and that the Israelites (and Jesus) had been black. These "converts" actually retained many of their old Christian practices and beliefs.

There's a similar story for the rise of black conversion to Islam in the 1920s. In most of the Muslim groups, the same old slave-era criticisms of white Christianity were used. The most famous Muslim group, for instance, the Nation of Islam, explicitly said that the Bible used by whites had been altered by whites to oppress black people. Only by converting and listening to the American Muslim prophets and Messengers could a person get a "true", "black" interpretation of the Bible. To this day, NOI members still quote from the bible to prove their religious points about coming apocalypses, moral principles, and the history of ethnic groups. Other religious concepts in the NOI and other black Muslim groups also clearly came from old slave Christianity (and sometimes hoodoo). Indeed, in many ways their identification Islam and their ostensible rejection of Christianity was only superficial. This is a testament to the power, influence, and identity with slave Christianity. It is also a testament to the fact that widespread religious change is rarely as dramatic of a transition as it may often seen from an outsider.

Sources: Olli Alho, The Religion of the Slaves: A Study of the Religious Tradition and Behaviour of Plantation Slaves in the United States, 1830–1865 (Helsinki: Academia Scientarium Fennica, 1976); Albert J. Raboteau, Slave Religion: The “Invisible Institution” in the Antebellum South, updated ed. (NewYork: Oxford University Press, 2004); George P. Rawick, ed., The American Slave: A Composite Autobiography, 19 vols. (Westport, ct: Greenwood Publishing, 1972); Fisk University, Social Science Institute, God Struck Me Dead: Religious Conversion Experiences and Autobiographies of Negro Ex-Slaves (Nashville: Fisk University, 1945); Fisk University, Social Science Institute, Unwritten History of Slavery: Autobiographical Account of Negro Ex-Slaves (Nashville: Fisk University, 1945); Eugene D. Genovese, Roll, Jordan, Roll: The World the Slaves Made (New York: Vintage Books, 1976); Savannah Unit Georgia Writers’ Project Work Projects Administration, Drums and Shadows: Survival Studies among Georgia Coastal Negroes (Athens: University of Georgia Press, 1940); Charles L. Perdue, Jr., Thomas E. Barden, and Robert K. Phillips, Weevils in the Wheat: Interviews with Virginia Ex-Slaves (Charlottesville: University Press of Virginia, 1992); Mechal Sobel, Trabelin’ on:The Slave Journey to an Afro-Baptist Faith (Westport, ct: Greenwood Press, 1979); Ras Michael Brown, African-Atlantic Cultures and the South Carolina Lowcountry (New York: Cambridge University Press, 2012); Yvonne P. Chireau, Black Magic: Religion and the African American Conjuring Tradition (Berkeley: University of California Press, 2003); Michael A. Gomez, Exchanging Our Country Marks: The Transformation of African Identities in the Colonial and Antebellum South (Chapel Hill: University of North Carolina Press, 1998); Patrick Bowen, A History of Conversion to Islam in the United States, Vol. 2 (Brill, 2017).