The liberal Christians who fought against slavery were going against Christianity due to being influenced by Enlightenment philosophers. Just because someone is Christian, doesn't mean they're always fighting for Christianity. There are also Christians who fought for women's rights and LGBT rights. Humans are complicated. They hold on to religions they don't fully believe in and take action that goes against those religions.
It's not that it's a religion of slavery. It's that it's a religion that permits slavery.
I agree with that much. Christianity was very popular among slaves for a bunch of reasons. My only point is that Christianity permits slavery. It was only during the Enlightenment that we started having widespread debate over the morality of slavery and there was a push to end it.
And as for the role of Atheists, that's actually hard to say. Back then, especially in America, Atheists were ostracized from society. So we don't really know how many Christian Abolitionists were secretly Atheist. It would be like saying there were no gay people fighting against slavery. If I had to guess, I'd say there were, we just don't know who they were.