Slavery's depravity is so obvious to us today that it obscures the collective mental conversion required to achieve this bit of moral clarity. This conversion entailed a titanic shift from seeing slavery as necessary for civilization (as the ancients believed) to seeing it as necessarily uncivilized. As one of humanity's most radical changes in social thinking, it is unsurprising that it took centuries to unfold.
Religion is often cast an impediment to slavery's moral stigmatization. The Bible treats slavery as a social fact (which it was at the time), and over the centuries religious leaders and communities held slaves and (often with qualifications) condoned the practice. One could legitimately argue that this was a major moral failing on religion's part. What is wholly illegitimate is the claim that secular wisdom dragged religion, kicking and screaming, into the realization that slavery was bad. For example, here's what University of Chicago biologist Jerry Coyne claimed in a July 31 USA Today op-ed piece on religion and morality: "Secular morality is what pushes religion to improve its own dogma on issues such as slavery and the treatment of women"
Wrong. Instead, monotheism provided the essential tools for making the colossal change in moral thinking outlined above.
Pagan authors occasionally condemned slavery. But they sorely lacked any compelling rationale for why slavery itself, rather than the maltreatment of slaves, was evil. This deficiency was all the more glaring in the face of powerful Platonic and Aristotelian arguments for both the necessity and naturalness of slavery. In discussing the ancient world's (pagan) critics of slavery, Harvard classicist Robert Schlaifer concluded: "Many writers protested against slavery as it was, without having the least doubt of the justice of the institution if properly applied" (Harvard Studies in Classical Philology, 47, p. 199).
Among the first communities (maybe the first) to prohibit slavery were two early first-century Judaic sects -- the Essenes and the Therapeutae -- and they did so for purely religious reasons (no secular coercion required). Both Philo ("Every Good Man is Free" 79) and Josephus ("Jewish Antiquities," 18.21; see J. D. Crossan's "The Birth of Christianity," pp. 445-449) tell us that these groups viewed slavery as unnatural and unjust, and therefore against God's laws. Biblical historian John Dominic Crossan sums it up this way:
"The Essene communities were radial attempts to live faithfully and fully the law of God, in justice and righteousness, in purity and holiness, when everyone around them, from their own high priests to their own people, was failing to do so" (p. 462).
This justice and righteousness was in part rooted in the uniquely monotheistic concept of "Imago Dei" -- that all humanity was created in God's image and therefore endowed with inherent value. It was Imago Dei that produced history's first unequivocal condemnation of institutional slavery -- that is, that slavery was flat-out, always and everywhere, morally evil, regardless of the slave's treatment. In his fourth sermon on the Book of Ecclesiastes given during Lent of 379, Gregory, Bishop of Nyssa, proclaimed:
"If he [man] is in the likeness of God ... who is his buyer, tell me? Who is his seller? To God alone belongs this power ... God would not therefore reduce the human race to slavery ... if God does not enslave what is free, who is he that sets his own power above God's?"
Gregory was not alone. In the fifth century, St. Patrick, himself a former slave, condemned the enslavement of free people and in 1102, St. Anselm (Archbishop of Canterbury) presided over a national ecclesiastical council which prohibited the slave trade: "Let no one hereafter presume to engage in that nefarious trade in which hitherto in England men were usually sold like brute animals."
America and the British Empire freed slaves in the 19th century (pushed in no small way by religiously inspired abolitionist movements). Popes started freeing slaves in the 15th century.
In 1435, Pope Eugene IV issued "Sicut Dudum," a papal bull ordering (on pain of excommunication) the emancipation of enslaved native Christians of the newly colonized Canary Islands and prohibiting future enslavement. By including both baptized Christians and "those freely seeking baptism" in the order, Eugene apparently intended to protect most if not all Canary Island natives (see J. Panzer's "The Popes and Slavery," p. 9).
In 1537, Pope Paul III issued the bull "Sublimis Deus," prohibiting New World slavery and declaring the full humanity of the Native peoples. Included under the bull's protection were "Indians and all other peoples -- even though they are outside the faith." There are controversies over whether Paul may have rescinded this bull a year later and the degree to which he may have conceded to slavery within Rome itself. Regardless, Catholic missionaries welcomed and employed Sublimis in their defense of natives, and future Popes routinely reaffirmed Sublimis as authoritative.
For example, in 1639, Pope Urban VIII (of Galileo fame) issued "Commissum Nobis," reaffirming Sublimis Deus and reiterating the penalty of excommunication for violators. On March 20, 1686, the Holy Office of the Inquisition issued "Instruction Number 230" ruling that the enslavement of black Africans was immoral and slaveholders were obliged to free and even compensate their slaves.
Why then did religious complicity with slavery persist despite these condemnations? One answer is that an argument for slavery's moral permissibility was possible within the Imago Dei framework. If someone was incapable of governing themselves (a slave to sin), then he or she would be better off under another's control. Thus, an "irrational savage" might benefit from being a "good" Christian's slave. Aquinas and other Church Fathers argued that this control, however, was only justified if the master's authority was exercised for the betterment of his subject ("Commentary on the Sentences," book 2, d. 44 q1, or see S. F. Brett, "Slavery and the Catholic Tradition," pp. 70-72). Theoretically, this might be defendable, but in practice (with large profits at stake) it proved far too easy to abuse.
Abolition's moral victory was laboriously slow and religion was not always an ally in that fight (U.S. Catholic clergy sometimes willfully misrepresented Papal pronouncements in defense of pro-slavery positions). The critical point, however, is this: Over the centuries, using its own moral precepts, religion developed the rationale necessary to categorically condemn slavery and took actions to implement that rationale. Indeed, over those same centuries, the secular world -- with great interests in profits and power -- often either ignored religious criticisms of slavery or pushed religion against abolition, not for it.
Supremely confident broad-brush pronouncements should always be viewed skeptically. History's complexities rarely confer them much credibility.