Being that one side believed in the bondage of human beings as a religious duty, and the other side believed that freeing human beings from bondage to be equally divine in nature, who's to say who is right? The Bible can be contradictory on this matter ("Servants, be subject to your masters with all fear; not only to the good and gentle, but also to the froward" vs. "Let my people go"). We can only use Reason at that point. Religion is a grand basis for the binding of societies together, to create a starting point for codified morality, but our ability to think and reason allowed human beings to create Consitutions and codified regulations that does not exist purely to gratify the spiritual. We continue to reason out that genocide is wrong, slavery is immoral, and that women should not be treated as inferiors to men...all of these things can be found in the Bible, the Koran, etc. However, only the most conservative of literalists this day and age agree with those repugnant belieft structures.
So where am I going with this? The basis for morality always shifts. The Bible is constantly reinterpreted. New religions and philosophies continually appear and rise. What makes this country great is that we have a Consitution, based loosely on Christian religious text, Roman legislative structure and the reasoning minds of some of the most impressive Intellectuals of the 18th to 19th Centuries. It is our guiding document that is not set in stone, but does not change with the whims of fractious and short-sighted humanity. Our laws come from it.