David said “...advocates of [the Bible/Koran] are the best authority of what its followers should be practicing...”
To some extent, I think this is a completely valid viewpoint, and that you are right to push in that direction. You should be held accountable for what you really believe, only what is really being taught.
However, you must realize that there is often a departure between what is taught and believed about the Bible versus what is actually in the Bible. For example, several hundred years ago, Christianity presented a fairly uniform front that hell was a place of eternal punishment, a lake of fire where those who rejected God were tortured forever. Today, you still have people who believe that, but you also have others who believe that hell is just the absence of God, or that the unsaved are simply terminated. All of these views are from advocates of the faith, but they can't all be correct.
Similarly, you would not have asked General Custer if the United States policy of slaughter and forced relocation of the Native Americans was justified. His view would be biased, for sure, presenting the best possible picture of the situation from the U.S. perspective.
Therefore, I submit to you that consulting Lewis or Wright about what to believe is one level removed from the truth. Our battle ground should not be the opinions of others, but rather the very words of God written in the Bible, and what the implications are regarding those words.
To advocate is to encourage support for something. That's different than mandating something. Having laws which regulate something instead of having laws which forbid something is a tacit approval of that something. What you have to recognize is that, at one point in time, God had rules regulating slavery, not prohibiting slavery.
You bring up an interesting point about Hebrew slavery, which was more like indentured servitude than true slavery, because it had a term limit defined by the seventh-year Jubilee. However, this is not the only kind of slavery which existed. You've got to read all of the text, and pay attention to the details.
Leviticus 25 is one of the chapters covering Jubilee. Leviticus 25:39-43 talks about how a poor Hebrew may sell himself, but must be released on Jubilee. It also says that he is not to be treated as a slave. In contrast is Leviticus 25:44-46, God instructs the Israelites to buy their slaves from other nations, and that these types of slaves are property which can be willed and inherited, precisely like the slaves of the United States were treated. In that last verse, Leviticus 25:46, you can see the double standard God has established in the treatment of slaves versus the Hebrew servants. The rest of that chapter explicitly explains that it is only, only, the Hebrew servants which are to be freed during Jubilee.
So whether or not Christianity today advocates slavery, God, Himself, at one time did.
Furthermore, God defined a double-standard of justice for slaves, such as Exodus 21:26-27. Where God would have prescribed eye-for-eye, tooth-for-tooth justice for free citizens, instead a slave just gets freed if his master knocks out a tooth or blinds him while his master receives no punishment. In Exodus 21:28-32, you see that if a wild bull kills a free man, the bull's owner will get put to death. But if the wild bull kills a slave, then the bull's owner just has to pay the slave's owner a fine.
So God not only advocated slavery, bu also advocated the treatment of slaves as second-class citizens, not worthy of the same justice which is afforded free men. That's not really what I would expect to see from a loving God. How about you?
I've spilled enough virtual ink for one argument. Let's cover oppression and genocide after we get slavery resolved.