"I assert most unhesitatingly, that the religion of the south..." — Frederick Douglass quote meaning

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 the slaveholders find the strongest protection. Were I to be again reduced to the chains of slavery, next to enslavement, I should regard being the slave of a religious master the greatest calamity that could befall me. For of all slaveholders with whom I have ever met, religious slaveholders are the worst. I have ever found them the meanest and basest, the most cruel and cowardly, of all others.