It’s an interesting thing in apologetics, and one that is on display in this thread: the arguments on the Christian side are usually very complicated, but the arguments on the nonreligious side are quite simple.
The reason for this is that the arguments from the nonreligious side make sense. The arguments from the Christian side do not, and that’s why they require such a lot of additional explanations to be tacked on to them. The Christians on this particular thread don't seem to be aware of this; they seem to think that what they are doing is taking the trouble to explain a difficult issue in detail. But it’s only a tricky theological problem if you’re a Christian, and their explanations only postpone the problem (this is a useful feature for apologists, however; as the arguments get longer and more complicated, it’s easier for them to hide the contradictions in them).
So: the problem is, if God is a good God, why does He endorse slavery?
On the one side, then, we have the atheist answer. God doesn't exist. The Christian God is just a character in a set of stories from the Bible. The pro-slavery parts of the Bible were written by men who thought that slavery was fine, and they enlisted God to back up their arguments.
As is so often the case, the tricky questions about “Why would God do that?” are easy to answer if you're not a Christian.
If, however, you are a Christian, then you have some explaining to do.
Let’s take a look at some of these explanations that Christians must use to try to resolve this issue.
1. “The Bible doesn't endorse slavery.” I include this for the sake of completeness. Some Christians do say it, but I think at this point in the thread we’ve established that it does.
2. “Slavery as described in the Bible was not all that bad.” Some Christians argue that there were mitigating circumstances. Masters weren’t allowed to actually kill their slaves, for example, or that some slaves entered into servitude of their own free will. This argument can't, of course, be taken seriously. The verses are quite clear. Masters could capture people, keep them against their will, keep their children against their will, and punish them with horrific cruelty if they wished.
3. “This was a form of “progressive revelation” is the other rationalization on display in this thread. Apparently God knew all along that slavery was bad, but He held His hand, allowed it to continue while introducing laws to control its worst excesses, and introducing special messengers such as Jesus to eventually lead to the abolition of slavery through better teachings.
This idea is wrong on many levels.
First of all, if it was God’s intention to end slavery, then He could have just done it. He’s God. The stories in the Bible are quite clear. He can do anything. He flooded the world, he cursed the Egyptians until they freed the Israelites, and if He wanted to stop people from holding slaves, He could have.
Secondly, even if there was some reason that God was unable to free the slaves, why didn’t He at least announce this plan of His? Say “Owning slaves is bad. I understand you don’t realise that yet, but it’s better to not have slavery, and one day you will abolish it, in accordance with My will.” The answer, of course, is that God approves of slavery (the Christian answer, that is, the only one they can honestly give. The atheist answer is that there is no God).
God could have laid down different laws in the Old Testament. Instead of saying “You can punish your slaves as much as they like, as long as it doesn’t actually kill them” how about “You can deprive your slaves of food and water for a certain period, as long as it doesn’t kill them” or “You can assign them extra work to do” or “You can have a system of different tasks, and give disobedient slaves the worst of them” or “You can have rebellious slaves trialled as criminals and sentenced to prison”. Or God could have just written in the Bible “It’s wrong to own people, wrong to make them work against their will, and wrong to punish your slaves”.
God could have done so very many things; and, if He is really God as Christians believe in Him (all-powerful, and the author of morality) then He would have done, just as He did when confronted with other things that He disapproves of. But He didn't.
I can just hear the response now: "Who are you to judge God?" But that's not what I'm doing. All I'm doing is letting the text speak for itself, and pointing out that any other readings of it simply do not make sense.
I understand that this is a problem for Christians who don't want to believe that the Bible is pro-slavery, but you do have a number of options. You could ignore it. You could attempt to rationalise it further (there really is no argument so absurd that an apologist will not use it); or, you could entertain the idea that maybe these conflicts within the Christian religion will make sense if you realise that God is imaginary.
