I think you might be making the mistake of giving religion too much fluidity with pro-slavery sentiment in the Americas. The attribution of certain texts, certain ideas, to support the economic system that slavery became has a much more varied past than simply "religion-->pro-slavery". From the reading I have done, religion was used in justification of slavery at different times, as numerous times it simply was not needed. In certain decades, in certain regions it was important for slave owners to Christianize slaves, to educate them so as to be proficient readers of the holy text and then at other times, such information was seen as dangerous, and barred. Remember, under God, all men are equal creations. How does that philosophic and religious sentiment mix with the Chain of Being, with its emphasis on rational hierarchy of all creations in nature? One of many conflicts, indeed!
Reasoning for the justification of slavery varied in time and place. Sometimes, the Enlightenment's emphasis on science, on logic, on order, created the perfect justification for the character of the American slave institution. Other times, the Enlightenment was the source of freedom. Same with religion. It had its liberalizing moments, and its restrictive moments.
On a side note: you could also take aim against your claim that it was merely fashionable to oppose slavery, as prior to that moment it was enforced. Again, it depended upon time, place, and to an extent, person. One could easily make the argument that until the heating moment in American politics, there was a sense amongst the American political establishment that at some point in time, the institution of slavery was going to be meet its expected death, but perhaps also that rapid change to a slave-free nation was a bad idea.
