At least slaves of the actual terrible form of slavery understood that their plight was objectively wrong (only true if God exists), then 90% of them prayed to him to end their plight (only true if his revelation made that possible, being that the Bible never shows God as liking slavery), and were actually delivered from their bonds primarily by Christians (only possible if faith existed), for reasons that are only true if God exists (objective equality, sanctity, and dignity of human life). At the very least in the 1860 US atheists did not have the validation, will, or power to deliver the slaves. I will let who the slaves selected for a deliverer and in what theological group form the deliverers came from conclude the matter to a finality neither you, I, nor the meaningless dissertations of moralists could ever do.