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Douglass wants to make sure that people don't get the wrong idea about his views on religion, so he writes a short appendix to set them straight.
Basically he's got three main points:
He's a dedicated Christian.
Slaveholders who say they are Christians are, in fact, not Christians.
Slavery and Christianity are opposed to each other.
There isn't much more to it than that, but it's still worth reading to get a sense for just how angry Douglass can get about religion. To drive the point home, he quotes lines from a poem by John Greenleaf Whittier called "Clerical Oppressors." The crux is, again, that Christians who own slaves are worse than hypocrites.
Finally, Douglass quotes a parody of a familiar hymn, "Heavenly Union," that he would sometimes sing at Abolitionist meetings. This is Douglass's Weird Al Yankovic moment. The original hymn was well known in white churches, but in Douglass's version, the joke is that none of the people singing it are actually part of any real union of humanity. When they go back home after church, they become slaveholders again.