If it does condone slavery, then why is it that all of Christianity is not promoting slavery?
Because mainstream religion has cherry picked the bits they like, and most christians don't even know it's in there?
And what's wrong with cherry picking? How else do you get the cherries? Once you move past the "written in stone" idea (which almost all Christians have), then it's just a matter of figuring which principles and practices still apply in the modern world. You can have a core of what is considered historical fact (Paul's summary of the core gospel truth in I Cor. 15, for example), and then the rest is a matter of contemporary application.
Well, I think we can at least say something good about the flexible nature of the Christians who have decided to get past the 'written in stone' idea. The problem is, despite their preference for not taking the Bible to be a 'written in stone' type of thing, the Bible itself says in several passages that it's the word of God, that it's meant to be authoritative, etc. In some cases (one, at least) the word of God was supposedly quite literally set in stone. So, either one believes that all the scriptures are the inspired word of God... or they don't. That's the problem right there, with picking and choosing what you like, and leaving what you don't. If Christians believed the Bible was completely, 100% human in origin, then sure, take only what you like -- the good bits.
Thing is, they believe the Bible to be divine. So when they gloss over or skip certain parts of it, what they're saying is either "I don't believe THIS part is from God", or "I don't really believe the whole thing is from God", or "whether it's divine or not, I don't buy that particular part". The Bible says God never changes, that he's the same yesterday, today, and tomorrow. So the argument can't even be made for "well, that was back then, and this is now." One can only logically assume that if 'God' said something and meant it then, he'd say it and mean it now -- otherwise, it would suggest that God changes with the times, just like we do.
Don't get me wrong -- I think it's great that humans change, that we're capable of change, and capable of rational, critical thought. I just think it doesn't do us much good to be picky and choosy about what we decide to think critically about.

But it's at least progress, to know that we're increasingly seeing the ancient texts as cultural, contextual, human documents. Which of course they are. At least in that sense, the Christians who also agree on this point seem to be taking part in a shift away from the belief that God is the essential author of the bible. They see that human flaws, bias, and culture snuck in there. Admitting that the bible perhaps isn't as inspired as previous generations insisted it was can only be a good thing -- because I'd HATE to think that God, if God existed, could honestly hold the "morality" found in so much of the Bible.