What does douglass say is a mere covering for the most horrid crime?

"the religion of the south"

I assert most unhesitatingly, that the religion of the south is a mere covering for the most horrid crimes,—a justifier of the most appalling barbarity,—a sanctifier of the most hateful frauds,—and a dark shelter under, which the darkest, foulest, grossest, and most infernal deeds of slaveholders find the strongest protection.