"The Christianity of America is a Christianity of whose votaries it may be as truly said, as it was of the ancient scribes and Pharisees, " --But woe unto you-- hypocrites! For ye shut up in the kingdom of heaven against men' for ye go in for yourselves""
"I learned it from watching you, Dad"
-1980's anti-smoking commercial
This is a quote of a quote, actually, but it is in this quote that Douglass lays bare the heart of what he's communicating in this appendix. The surface message is clear enough, Christianity's core values are lost on Christians in America. In Douglass' eyes, there is no greater evidence of that than in America's embracing of slavery. When you boil down Christ's message its most basic components, you are left with something pretty simple, God loves everyone, and therefor, we must love each other in the same way he loves us. Frederick is dead on in pointing out the gigantic conflict that exists in loving one's brother, while you also enslave and abuse hundreds of thousands of them.
The idea of the hypocritical Christian did not start here, and has also been echoed throughout the ages. The rampant success of the religion had already proven that people would do a lot of crazy stuff in the name of faith, including but not limited to killing for, and dying for. In the early days of Catholicism, there were plenary indulgences, the idea that you can buy your way into Heaven. This meant using earthly money to absolve you of your earthly sins, as a bribe to the priests, who supposedly had a more direct line to the Almighty, and who could put in a good word for you, for the right price. Whole wars were fought, lined with soldiers who felt safe charging into battle because they knew they'd be dying for a holy cause, and that their place in Heaven was secure.
This is where the America angle fits in. After all, America started out as nothing but a big European colony, made up of people who lived and died by the corrupt clergy. Using religion as a justification of terrible things like slavery, witch burnings and the subjugation of women seems in this context to merely be a sad, but natural progression. To invoke a tired cliche "We learned it from watching you, Dad."
Frederick recognized it as an American problem, but that doesn't mean it's true for the whole world as well. If we use religion as a means to harm and enslave, we miss out on the whole point. Sadly, this problem did not end with slavery in the United states. To this day, all over the world, there are places where slavery is still rampant, sometimes even tolerated or encouraged. In many cases, it is still sanctioned by religious doctrine. Frederick Douglass helped to strike a major and victorious blow for human rights with his writings, but even he knew that "until all of us are free, none of us are free".