I'm currently reading a new publication about Southern ministers and their defense of slavery. They knew that Scripture didn't condone slavery or comdemn it -- it's simply accepted as a social norm, as in Eph. 6:5-9. In contrast, many Northern ministers condemned all slaveholders as guilty of gross sin. Despite the lack of scriptural support, they believed slavery to be against the 'spirit' of Christianity. Of course people argue along the same lines on other topics today.
The more things change....