Someone once said that the Bible needs no defense. It's like a lion; turn it loose and it will defend itself. As true as that is, it doesn't help the believer who is defending biblical standards against the perversions of the day. In arguing for the biblical way, one often encounters a mocking fool who argues against the whole of scripture by attacking a part that is out of fashion. One common line of mockery goes, "If I'm going to follow the Bible, how should I treat my slaves?"
While the subject is too complex to argue with a fool, it is important and should be discussed openly rather than being avoided out of embarrassment. First it should be noted that the abolition of slavery in both Britain and the United States was driven by Christians and was regarded as a moral duty rooted in Scripture. In fact, we'll see that Biblical slavery contains the seeds of its own demise.
There are two sorts of slavery allowed to Israel in the Torah, slavery for Hebrews (Ex 21) and slavery for gentiles (Lev 25.44). In the case of Hebrew slaves, the rules are quite specific and strict. They can't be treated ruthlessly (Lev 25) and they must be released at the end of the sixth year. In prescribing the treatment of slaves, God repeatedly cautions Israel that they had been slaves in Egypt and that He freed them.
So, if the references to slavery are replete with reminders of its horrors, why is it allowed at all? I will suggest that the answer lies in redemption. Strictly speaking, a Hebrew could only find himself enslaved if he had failed in some rather large way. He could be sold into slavery by a court to pay off debts or to make restitution for something destroyed or stolen. In other words, healthy, productive members of society did not become slaves. Ideally, once they were enslaved, they would spend several years in an environment where they were forced to learn the habits of their more successful masters. At the end of the sixth year, the slave was then supplied with enough resources to get started in a new career and sent out free.
When Israel fell short of this ideal (Jer 34.8-22), God was quite upset with them. But the fact that humans fall short of the standards set by God is nothing new and human failure doesn't invalidate the Torah.
The rules for gentile slaves were different. There there was no prescribed time of release and it appears on the surface that gentiles could be enslaved eternally. But perhaps not. Given the redemptive purpose of Hebrew slavery, is it possible that the same purpose exists for gentiles? Once a gentile had come into a Hebrew household and (perhaps) become a worshiper of God, did he remain an alien? Indeed, a variation on this question was one of the animating forces among Christian abolitionists in the 19th century. There the question was, 'How can one justify enslaving his brother?'
As with everything else in Scripture, God's understanding is much deeper than ours. We, living in the world, are constantly blown around by the winds of human pride and fashion. Two hundred years ago slavery was fashionable in the Christian world (it is still so in the Moslem world), with the rise of humanism, racism became fashionable a hundred years ago, today sodomites are fashionable. Through all this, God's word has remained steady, and rather than being an embarrassment for the believer, Biblical slavery seems to have served a redemptive purpose.