I find, since reading over the foregoing Narrative, that I have, in several instances, spoken in such a tone and manner, respecting religion, as may possibly lead those unacquainted with my religious views to suppose me an opponent of all religion. To remove the liability of such misapprehension, I deem it proper to append the following brief explanation. What I have said respecting and against religion, I mean strictly to apply to the slaveholding religion of this land, and with no possible reference to Christianity proper; for, between the Christianity of this land, and the Christianity of Christ, I recognize the widest possible difference--so wide, that to receive the one as good, pure, and holy, is of necessity to reject the other as bad, corrupt, and wicked. To be the friend of the one, is of necessity to be the enemy of the other. I love the pure, peaceable, and impartial Christianity of Christ: I therefore hate the corrupt, slaveholding, women-whipping, cradle-plundering, partial and hypocritical Christianity of this land. (Appendix.1)
Douglass wants to be completely clear that he isn't against religion or Christianity in general: he's just against the kind of religion that justifies slavery. In fact, he also doesn't believe that "the slaveholding religion" is a religion at all.