A few initial thoughts putting the question of why the Bible does not offer an outright condemnation of slavery in context. There is a simple answer sometimes given which has more than a kernel of truth ("slavery was such an integral part of societies in antiquity that calling for the abolishment of slavery in the ancient world would have been akin to calling for the abolishment of money in our world")* but it is worth reflecting on this more broadly.
(1) Slavery is not a natural part of creation order but a social-historical institution. It is possible to be born into slavery but the Bible offers no justification for the belief that some people are born to be slaves based on their race** (as, e.g., in much of Western colonialism) or class (as maybe in the Hindu caste system in the East). There is no instruction within the Bible to uphold slavery as if the institution was necessary for an ideal ordering of the world.
(2) The institution of slavery can and should be regulated. Not all slavery is governed by rules. In the ancient world as well as in Islamic and European and American slave trade, slavery was often linked with kidnapping. This is condemned in the Bible (e.g., Exodus 21:16; Amos 1:6, 9). The people of God under the old covenant were given rules to regulate the institution (Exodus 21:1-11; Leviticus 25:39-55; Deuteronomy 15:12-18) and were commanded not to return fugitive slaves to their masters but welcome them within their own community (Deuteronomy 23:15-16).
(3) One corollary of the first two points is that slavery was not a monolithic entity within the biblical world. It can take many and various forms and there are fuzzy boundaries. "Given the historical significance of the Atlantic trade it is not surprising that the dominant stereotype of slavery is that of the New World Afro-American plantation system, a stereotype in which 'slavery is monolithic, invariant, servile, chattel-like, focused on compulsory labour, maintained by violence, and suffused with brute sexuality' (Kopytoff 1982:214). Yet examples from different times and places of what is usually taken to be slavery reveal a great variation in both the type of servitude slaves experienced (a common difference often being noted between domestic and chattel slaves), and the political and economic systems in which the institution existed." (P. Thomas, "Slavery," Encyclopedia of Social and Cultural Anthropology, ed. A. Barnard and J. Spencer [Taylor & Francis, 1996], 509-10, p. 509) Many slaves within the Greco-Roman world carried out sensitive and highly responsible tasks, e.g. as doctors and accountants, teachers and bailiffs, sometimes being better educated than their masters, and emancipation was a real possibility for the majority of urban and domestic slaves.
(4) The assumption within the Bible is that people holding ownership rights over other people is not in and of itself immoral. This is the nub of the issue for us today. It is arguably first of all a philosophical clash relating to notions of freedom and self-determination. In practice the biblical world and ours are not quite as far apart as it appears at first because on the one hand ownership was not total and absolute but governed by God's decrees and on the other hand individual liberty in the contemporary world is also sometimes severely restricted and in the same circumstances that led to slavery in the ancient world, namely economic hardship. Again, the experience of New World slavery misleads some into thinking that slavery is defined by treating people as property in a way which denies their personhood but if one were to accept this definition much of ancient slavery within and outside the Bible would have to be called something else. In the ancient world it was possible to consider slaves both property and persons with legal rights, e.g. the right to appear as witnesses, plaintiffs or defendants in court and to own property, including slaves.
(5) Slavery within the ancient world was gendered. In particular, masters seem to have been universally male or nearly so and female slaves were treated differently from male slaves. A woman sold into slavery regularly became a concubine or secondary wife in the process with attendant obligations and rights. In particular, this seems to have been a way to provide for a woman when a father could not provide a dowry. Thus the issue relates to the question of patriarchy and the relationship between the institution of slavery and the functioning of kinship structures. Masters also bought female slaves to give in marriage to their male slaves.
(6) Legally regulated slavery within ancient societies was regularly related to either avoiding or responding to economic hardship. P. Garnsey observes: "This points to a paradox at the heart of the slave system. Slavery is the most degrading and exploitative institution invented by man. Yet many slaves in ancient societies (not all, not even all skilled slaves, a class that included miners) were more secure and economically better off than the mass of the free poor, whose employment was irregular, low-grade, and badly paid...It was not unknown for free men to sell themselves into slavery to escape poverty and debt, or even to take up posts of responsibility in the domestic sphere." (Ideas of Slavery from Aristotle to Augustine [CUP, 1996], 5)*^
(7) Slavery was often a consequence of war. This is sometimes considered the chief source of slaves in ancient societies but this seems unlikely for Israel and Judah. The regulations are again gendered and varied depending on the course of the battle. A city that accepted terms of peace seems to have become subject to serfhood as a vassal entity rather than slavery, while military confrontation led to slavery as an alternative to death (cf. Deuteronomy 20:10-15; 21:10-14).
Much more could be said, especially about the way in which Christ elevates slaves in such a way that the abolishment of slavery arguably becomes at one and the same time less urgent and inevitable in the long run but for now it is worth noting by way of summary that the Bible does not condone slavery in all its various forms. It specifically condemns theft and allows for slavery only within a certain framework. The biblical instructions assume (a) that slavery is not in and of itself immoral but can become so, and (b) that there was not always a ready, less de-humanizing alternative to slavery. Today we have found other ways of dealing with people who cannot pay their debts (prison) and children who are an economic burden on their parents (abortion, adoption).
* Cf. "The institution of slavery was taken for granted not only by the free persons but also by the slaves themselves, who never demanded its abolition. Therefore ideology of the [Ancient Near East] contains no condemnation of slavery or any protest against it." (M. A. Dandamayev, "Slavery (ANE)," ABD 6:58-62, p. 61) -- S. S. Bartchy observes that "ancient Greece and Rome are two of only five societies in world history which seem to have been based on slavery." He also notes: "It must also be stressed that, despite the neat legal separation between owners and slaves, in none of the relevant cultures did persons in slavery constitute a social or economic class...Slaves' individual honor, social status, and economic opportunities were entirely dependent on the status of their respective owners, and they developed no recognizable consciousness of being a group or of suffering a common plight...For this reason, any such call as 'slaves of the world unite!' would have fallen on completely deaf ears." ("Slavery (Greco-Roman)," ABD 6:65-73, p. 66)
** "Slavery, which both long preceded and continued after the emergence of race, assumed a new dimension with global racialization. Before the 1400s, slavery was widespread in state societies, but its victims, either recruited internally or from neighbouring groups, were largely physically indistinguishable from slave-holders; slavery was a status that, as fortunes changed, might be held by anyone." (R. Sanjek, "Race," Encyclopedia of Social and Cultural Anthropology, ed. A. Barnard and J. Spencer [Taylor & Francis, 1996], 464-64, p. 463)
*^ See also the various references to debt slavery in R. Westbrook and G. M. Beckman, A History of Ancient Near Eastern Law, 2 vols (Brill, 2003) and S. S. Bartchy's comment: "Furthermore, by no means were those in slavery regularly to be found at the bottom of the social-economic pyramid...Rather, in that place were those free and impoverished persons who had to look for work each day without any certainty of finding it (day laborers), some of whom eventually sold thesemlevs into slavery to gain some job security." ("Slavery (Greco-Roman)," p. 66)
