The notion of "false" and "real" Christianity occupies a great deal of importance in Douglass' work. The fact that many of the slave owners, perpetrators of abuse and cruelty, were actually church going people on Sunday is where the hypocrisy resides. Douglass, who does believe in the tenets of pure Christianity, cannot understand how people can sit in the pews on Sunday and listen to the teachings of Jesus who stresses to "love thy neighbor" and then in the afternoon, whip slaves, break up families, and engage in rape of slaves on both physical and emotional levels. In this light, Douglass parallels the ringing of the auction and church bells as one in the same. At the same time, the larger implication is where the Church was during the time of slavery? If it was silent, then Douglass implies the it was complicit in the dehumanization, death, and suffering of millions. If it spoke out, it certainly was not heard, indicating the lack of faith amongst its professed followers.
I think that the best place to find the answer to this question is in the Appendix. There, Douglass explicitly addresses this question.
He says that American Christianity is not Christian at all. He says that the church and slave holding go hand in hand and he seems to blame the church for it.
What he says is that they church and its members are so blind (willfully so) that they are willing to accept the idea that slaves should be slaves and there is nothing wrong with slave holders.
So he says that the church condones slavery and makes it appear moral and that by doing so the church helps slavery survive.