One would be hard pressed to deny that slavery as an institution was widely accepted in the ancient world. Both the Old and New Testaments, participating in the cultural consciousness of their day, also appear to accept slavery as an institution (cf. Exodus 21:1-32; Lev 25:39-55; 1 Cor 7:21ff; Philemon; 1 Pet 2:18). (By the way, I am very open to any readings of these texts that would argue otherwise. If you happen to have commentary suggestions or know of exegetically-based arguments that have been published in scholarly essays, please send them my way). However, I find it not insignificant that at least two premodern Christian theologians/philosophers, concluded (1) that slavery as such is immoral (Scotus) and (2) that slavery is never a “natural” condition but one that has arisen as the result of sin (Augustine). For example, in De civitate Dei, St. Augustine says,
The first cause of servitude, therefore, is sin, by which man was placed under man in a condition of bondage: a condition which can come about only by the judgment of God, in Whom there is no injustice.
Augustine goes on to state,
By nature, then, in the condition in which God first created man, no man is the slave either of another man or of sin. But it is also true that servitude itself is ordained as a punishment by that law which enjoins the preservation of the order of nature, and forbids its disruption. For if nothing had been done in violation of that law, there would have been no need for the discipline of servitude as a punishment. The apostle therefore admonishes servants to be obedient to their masters, and to serve them loyally and with a good will, so that, if they cannot be freed by their masters, they can at least make their own slavery to some extent free [cf. Eph 6:5]. They can do this not by serving with cunning fear, but in faithful love, until all unrighteousness shall cease, and all authority and power be put down, that God may be all in all [1 Cor 15:24, 28].
In addition to Augustine, Duns Scotus in article 1 of Ord 4, d 36, q 1, argues in stark contrast with his ancient (e.g., Aristotle) and early medieval predecessors (excepting Augustine) that slavery (as described by Aristotle in bk. I of the Politics) is incompatible with natural law, is “not good but bad for the slave,” and is introduced “only by positive law.” Scotus goes on to say that there are only two instances in which this kind of slavery can be just: (1) voluntary servitude (e.g. to pay a debt) and (2) in the case of hardened criminals who might otherwise harm themselves or others. Yet, he says that (1) is “foolish” and still may go against the law of nature (Wolter/Frank, Duns Scotus on the Will and Morality, pp. 325-6).
I often hear the claim made that convictions such as (1) human beings should not be considered the “property” of another human being and (2) slavery per se is morally reprehensible are simply modern/postmodern sensibilities created and propagated by political liberalism (which is not a jab at political liberalism). I have to admit that I am deeply suspicious of this claim and find it rather unconvincing. After all, there were at least two premoderns (Augustine and Scotus and imagine many others of which I am unaware) who claimed that slavery was un-natural (contra Aristotle) and that it violated natural law. (Augustine does, however, seem to offer more of a justification for the institution that might not be in the end very helpful for seeking to abolish slavery. Scotus’s position, in contrast, might provide a stronger argument for the injustice and moral wrongness of all forms of slavery wherein one human “owns” another as property).
Among other questions that one might raise, what role do the following play: (1) cultural blindness (and I’m not denying cultural blindness and biases in our own day) and (2) an oppressive system that prohibits or seeks to suppress voices which speak against the acceptance of slavery as an institution? Relating these questions to the texts of Scripture, should we understand what appears to be an acceptance of slavery as a given state of affairs as an example of cultural blindness on the part of the human authors of Scripture? (I’m not willing to say that the Bible explicitly promotes slavery). Lastly, if it turns out that slavery in the ancient world (OT times and the Greco-Roman period) is in many ways significantly different from the various manifestations of slavery in the modern world, might these dis-analogous aspects provide a basis for a strong condemnation by (modern/postmodern) Christians of any form of slavery (or related oppressive practices) driven by racial hatred or an elitism in which one group considers itself ontologically superior to another and thus denies equal educational, employment, housing and so on to the putative inferior group?
 Augustine De civitate Dei, 19.15; ed. and trans. R.W. Dyson (Cambridge: Cambridge University Press, 1998), 943.
 De civ. Dei., 943-44.
 For example, many New Testament scholars (S. Scott Bartchy) stress that some slaves of the Roman period were wealthy, well-educated, owned other slaves, and were more economically secure than many free peasants. In addition, Bartchy claims that slaves in the NT period did not constitute a social class but rather a “juridical class.” Bartchy adds, “In outward appearance it was usually impossible to distinguish among slaves, freedmen and free persons. Neither the slave’s clothing nor his or her race revealed a legal or social status. Patterns of religious life, friends, or work did not separate slaves from freed persons or freeborn workers” (“Slavery” in Vol. 4, Q-Z of The International Standard Bible Encyclopedia (Grand Rapids: Eerdmans, 1988): 544). A number of other scholars, however, argue vehemently against such overly positive presentations of ancient slavery (cf. Peter Garnsey, Ideas of Slavery From Aristotle to Augustine).