It's not actually a religion. Frederick Douglass used the term "slaveholding religion" to object a religion that justifies slavery or those who used Christianity to justify atrocities.
Douglass isn't against religion or Christianity. He used ironic language to condemn the two-faced “piety” of his oppressors.
As per Litcharts.com, Douglass himself appears to possess a great deal of faith in a more humane Christianity; he writes, “I love the pure, peaceable, and impartial Christianity of Christ: I therefore háte the corrupt, slaveholding, women-whipping, cradle-plundering, partial and hypocritical Christianity of this land.”
Ultimately, through his narrative Douglass is making the case that slavery is incompatible with true Christianity, and in doing so making the case against slavery on religious grounds.
Frederick Douglass escaped slavery to become one of its most formidable opponents. When working on his autobiography "Narrative of the Life of Frederick Douglass, An American", he was a man whose life goes far beyond the legend.
Here are five things you might not know about him shared by CNN.