Your pessimism is justified, but it should be said the evangelicals have a long history of opposition to slavery. The failure of the evangelicals is not a lack of good intentions, but a lack of sufficient influence. (When I speak of the evangelicals, I refer to the preachers, not average churchgoers.)

I am reading a book about the rise of radicalism in early 19th century Upstate South Carolina (Lacy K. Ford. Origins of Southern Radicalism: The South Carolina Upcountry 1800-1860). Initially, evangelicals preached against slavery, but people were not listening, so the evangelicals trimmed back their message and only tried to persuade people to treat slaves humanely.

In the romanticized view, slavery was a long-established tradition practiced by aristocrats who were raised from birth to be enlightened, kindly masters.

None of this was true about upstate South Carolina. There was very little slavery there until the invention of the cotton gin made it profitable to raise short-staple cotton. This created an economic bubble, accompanied by the self-deceptions that characterize all economic bubbles. The residents of the upstate had been uneducated, underchurched and poor. Slavery was the only route available at the time by which a poor man could make himself rich (at the expense of the slaves, it goes without saying). Men would marry rich women to obtain the funds to buy slaves. Self-justification grew to the extent that people convinced themselves that King Cotton was a superior economic system to full industrialization.

The truth is that slavery in the Upstate was a new institution, which was created with the full knowledge that it was going against the most enlightened thinking of the time. It was all about money and self-interest. Being dependent on the invention of the cotton gin, slavery in the Upstate was a product of the industrial revolution. It was the agribusiness of its time. It can be understood as an example of the abuses of early unregulated industrial capitalism. Slavery was profitable because the economic system supported it. From an amoral, purely economic standpoint, slavery was good for business. This is not a justification for slavery, but a condemnation of unregulated capitalism.

With the benefit of hindsight, this could not have happened if the founding fathers had done the right thing and ended slavery when they wrote the United States Constitution. If some states were unwilling to join such a union, the union should have been created without them.

According to Edward Ball, in his book, Slaves in the Family, the idea of the kindly master was a 19th century innovation, due to the influence of the evangelicals. The idea of the kindly master probably did not penetrate very deeply, but the evangelicals do deserve credit for trying to spread the idea.

The evangelicals, however, were not able to overcome the fact that slavery was throroughly integrated into the American economy. A valuable book on the economics of slavery is The Meaning of Slavery in the North, edited by David Roediger and Martin H. Blatt, summarized at :http://www.cr.nps.gov/history/hisnps/NPSBooks/slavery.htm