by at least half a dozen others claiming that Douglass' religion had nothing to do with his anti-slavery and other human-rights work.

Religion had something to do with it. Just like it had something to do with pro-slavery positions. The God of the Bible very clearly advocated and sanctioned slavery (and genocide, rape, torture, etc.).

Trying to sell Christianity as an anti-slavery religion still makes no real sense to this day. But anti-slavery resonated with logic, rationality, secular morals and enlightenment ideals, and consistency has never mattered much in religion anyways. A person takes what they like and ignores what they don't.

The real point is that a belief in supernatural beings is not necessary to be a good person, but it can lead otherwise rational, empathetic people to take very hateful and bigoted positions, like the continued bigotry against the GLBT community, even by some of the first anti-slavery churches. That's the danger of believing things not based on rationality, but on a book like the Bible.

That he would not have been anti-slavery or supported other human-rights works had it not been for his religion? If that is so it sounds to me like you are selling a great man rather short. But I don't think that is what you are saying.

If what you want to say is that his religion kept him motivated, then that is fine and it deserves credit. However, by the same token, it deserves blame for motivating pro-slavery proponents and allowing them to think they were justified in their beliefs.

Go ahead and argue against that.

His anger and moral conviction about the justice of his cause shine through convincingly in sentence after memorable sentence

I still remember the effect this had on me, reading it for the first time in a course on the history of American ideas.

Smote me right betwixt the eyes, much as I imagine it did his original audience.

Very, very powerful.

Ever read any of David Walker?

Great stuff.