Why did God permit slavery?
There are Old Testament laws and principles that don't have validity today, and there is a redemptive-historical flow in the Bible that accounts for why some things were both commanded and permitted earlier that aren't now.
Part of this flow is that the people of God in the Old Testament were a political and ethnic reality with God as their King, and later with an earthly king. God ordained in those circumstances that his people immediately exercise some of his rights and his judgments upon the people.
Therefore you have the entire annihilation of the Canaanites by Joshua and his army with brutal, universal destruction. That's because God said in Deuteronomy, "I'm punishing these people for their sins. It's not your righteousness, O Israel, that is bringing this about. It's their sins that are bringing this about." So he was using his people as an instrument for his own judgment. In the context of a theocracy that was legitimate and right for God to do, even though the people themselves may have been sinful in the execution. And it was similar with things like slavery, God saying, in essence, "You're my people. Those people I have a right to judge. You may own them," and so on.
Now here comes Jesus, and he undoes much of the Old Testament law. In fact, I think he undoes all of it as law, according to Romans 7:4 where it says that we died to the law so that we may belong to another. And the reason he undoes it is not because it was wrong under those circumstances to do what he said to do. Rather, with the coming of Christ, his rejection of an earthly kingdom, and the establishment of a spiritual kingdom, Jesus says that the kingdom will be taken away from the Jews and be given to a people who bear its fruits, that is, the church.
Now the Kingdom of God isn't political, ethnic, or geographical. It has no king. It is a church made up of all ethnicities from all over the world, and therefore it has a very different witness to bear in the world. It is a "go tell" religion, not a "come see" religion. It is a religion of binding Jew and Gentile of all stripes together in the blood of Christ, not the religion of a Jewish phenomenon performing wonders that the world could look on and maybe get saved.
And under that new circumstance—of a spiritual dying and rising Messiah who reigns from heaven (with no political dimensions to his faith on the earth), and a desire for all the peoples to come together (not just one ethnicity), and a desire that the command of love take on a whole new universal scope (because the very essence of the incarnation and the death of Christ was to love your enemies)—a whole string of Old Testament processes, procedures, and commandments go by the wayside as part of the old system and not part of the new.
Now with regard to slavery in particular, it seems to me that the New Testament also causes us problems, because it tells slaves to be submissive to their masters and for masters not to threaten their slaves. So it's operating with a system of slavery that was given at that time.
However, the commands that are given and their context, I think, contain all the seeds for undoing anything like what we had in this country, for example, with the owning of human beings and its racial basis.
The book of Onesimus (Philemon) is the book that is brought forward most often—and rightly, I think—to show that Paul was sowing the seeds to explode the whole situation of slavery. Onesimus himself was a slave when he got converted. Paul sent him back to Philemon who had been his master, and he said, "I am sending him back as a brother. Honor him." I think that kind of spiritual dynamic is intended to explode the system.
Another thing to explode the system is when Paul says to masters, "Do not threaten them, remembering that you too have a master." So he puts the command of neighbor-love—do unto others as you would have them do unto you—in the place of the right of the master to threaten. And if you don't threaten, what do you do? You win by love, and that transforms slavery into employment.
So I think it's not wrong that the Christians in America moved from a justification of slavery to a justification of the abolition of slavery, and that it was long and hard in coming. The biblical principles that were used to undermine the Old Testament's own speech about slavery was appropriate. It's right to say that there are changes that come about in the process of redemptive-history which make some laws in the Old Testament no longer appropriate or relevant at all in the New Testament.