What is Frederick Douglass's overal claim in The Narrative of the Life of Frederick Douglass??What evidence does he use to support his claim?

Douglass' primary claim in his narrative is that a nation predicated upon individual freedom cannot allow slavery to exist.  His first hand accounts of the cruel nature of slavery and the thought processes of the various slave owners help to substantiate his assertion.  It is difficult to challenge the accuracy of the first hand account offered.  At the same time, Douglass' exploration of Christianity, and its hypocrisy when used to justify slavery, and the idea that the promise of freedom cannot be realized when a group of people are enslaved, helps to bolster the claims that slavery is America's "original sin."