But the spread of Christianity did little to stop the spread of slavery because too many Americans made money off slavery, the documentary shows. The wealth produced by slavery transformed the United States from an economic backwater into an economic and military dynamo, says Gilpin, also author of "John Brown Still Lives!: America's Long Reckoning With Violence, Equality, and Change."

"All the combined economic value of industry, land and banking did not equal the value of humans held as property in the South," Gilpin says.

Many Americans hated abolitionists because they saw them as a threat to prosperity, says David Blight, a Yale University historian featured in "The Abolitionists."

"They wondered if you really did destroy slavery, where would all of these black people go, and whose jobs would they take," says Blight.

The South wasn't the only region that profited off the slave trade. Abolitionists faced some of their most vicious opposition in the North. New York City, for example, was a pro-slavery town because it was filled with bankers and cotton merchants who benefited from slavery, Blight says.

"Jim Crow laws did not originate in the South; they originated in the North," Blight says.

The lesson: Don't reduce the issue of slavery to racism. Follow the money.

No. 3: Flawed reformers

The historian Henry Louis Gates Jr. once said that black abolitionists used to say that the only thing white abolitionists hated more than slavery was the slave.

"The Abolitionists" reveals that some of the most courageous anti-slavery activists were infected with the same white supremacist attitudes they crusaded against. White supremacy was so ingrained in early America that very few escaped its taint, even the most noble.

The documentary shows how racial tensions destroyed the friendship between two of the most famous abolitionists: Frederick Douglass and William Lloyd Garrison. Garrison was the editor of an abolitionist newspaper who convinced Douglass that he could be a leading spokesman against the institution that once held him captive.

Erica Armstrong Dunbar, a history professor featured in the film, says some abolitionists were uncomfortable with interracial relationships. They wouldn't walk with black acquaintances in public during the day, and refused to sit with them in church.

Lesson: Racism was so embedded in 19th century America that even those who fought against racism were unaware that it still had a hold on them.

"The majority of abolitionists did not believe in civic equality for blacks," Dunbar says. "They believed the institution of slavery was immoral, but questions about whether blacks were equal, let alone deserved the right to vote, were an entirely different subject."

No. 4: Lincoln the "recovering racist"

Tell some historians that "Lincoln freed the slaves" and one can virtually see the smoke come out of their ears.

"Please don't get me started," Dunbar says after hearing that phrase.

"There's this perception that good old Lincoln and a few others gave freedom to black people. The real story is that black people and people like Douglass wrestled their freedom away," Dunbar says.

Historians still argue over Lincoln's racial attitudes. The historian Henry Louis Gates Jr. once called him a "recovering racist" who used the N-word and liked black minstrel shows.

Others point to the public comments Lincoln made during one of his famed senatorial debates with Stephen Douglas in 1858 when he said, "There is a physical difference between the white and black races which I believe will forever forbid the two races living together on terms of social and political equality.