1.) Christians in liberal societies have argued about slavery in the Old Testament since the beginning, and the arguments proved both difficult to resolve and prone to violence, as is clear from Mark Noll’s The Civil War as a Theological Crisis.
2.) No plausible reading of Scripture allows us to see slavery as a malum in se. Then again, no plausible reading of the Thirteenth Amendment allows this either:
“Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.
Notice the except clause. If slavery were an intrinsic evil, it could never be a punishment, since punishments are just. This is why prisons involve mandatory labor under threat of punishment, and the only difference between this and slavery is between a term defined and its definition.
3.) The penal slavery or war slavery in scripture can be explained in terms analogous to how we would explain our own slavery systems in the contemporary US, and presumably we would evaluate it as moral or not on those terms, adjusted for historical and cultural norms. This explains a large part of biblical slavery, but not all. What about the rest?
4.) One fails to understand God as creator if he fails to see that man by nature is literally a divine chattel. God has full abusus rights over human persons, just as we have abusus rights over any animals, plants, and inanimate things. If God makes me your slave, or puts his authority behind a system by which it comes to pass, then I am your slave in fact, and by your moral right. All this is implicit in our calling God Lord. In liberal society, of course, Christians still say “Lord,” but the force of the word has been watered down to, at most, something like “boss,” and its common usage is often indistinguishable from a word like “buddy.” The whole notion of God as Lord is intelligible only as offensive, but this yields a theology utterly incompatible with scriptural theology, or with the truth of the matter.
5.) Since everyone allows for morally acceptable slavery in principle, and to some extent in practice, the only question is what falls under this moral umbrella. We contemporary Americans limit the practice to penal slavery (and presumably war slavery) but even if we said everything beyond this was immoral, it would not address the morality of divinely sectioned slavery that in fact went beyond this. I’m already a divine slave by nature (as opposed to grace) and so God is within his rights to treat me as one, or even to delegate his authority over me to others. If I’m justly owned I can be justly given or sold.
6.) Our inability to see this is where our liberal indignation over slavery becomes theological error. We have come to see ourselves as not even God’s slaves by nature. This is both false and rules out our ability to give astonished thanks for being, by grace, “no longer slaves, but friends” or to rejoice that God has sent the Spirit of his Son into our hearts, crying, “Abba! Father!” making it so we are no longer a slave, but a son.