Here is an interesting except from a book review by Avery Cardinal Dulles, S.J. in First Things. The whole article (which I recommend reading) is a fascinating window into the sophisticated nature of Catholic theology. (Emphasis mine)
‘Slavery was practiced by almost every known society until modern times. Throughout the biblical era, Noonan shows, slavery was taken as a given, although the Israelites practiced rather mild forms of slavery and did not permanently enslave their compatriots. Jesus, though he repeatedly denounced sin as a kind of moral slavery, said not a word against slavery as a social institution. Nor did the writers of the New Testament. Peter and Paul exhort slaves to be obedient to their masters. Paul urges Philemon to treat his converted slave Onesimus as a brother in Christ. While discreetly suggesting that he manumit Onesimus, he does not say that Philemon is morally obliged to free Onesimus and any other slaves he may have had.
For many centuries the Church was part of a slave-holding society. The popes themselves held slaves, including at times hundreds of Muslim captives to man their galleys. Throughout Christian antiquity and the Middle Ages, theologians generally followed St. Augustine in holding that although slavery was not written into the natural moral law it was not absolutely forbidden by that law. St. Thomas Aquinas, Luther, and Calvin were all Augustinian on this point. Although the subjection of one person to another (servitus) was not part of the primary intention of the natural law, St. Thomas taught, it was appropriate and socially useful in a world impaired by original sin.
The leaven of the gospel gradually alleviated the evils of slavery, at least in medieval Europe. Serfdom did not involve the humiliation and brutality people today ordinarily associate with slavery. Moral theologians recognized that slaves, unlike mere chattels, had certain rights even against their masters, who no longer had over them the power of life and death, as had been the case in pagan antiquity.
For St. Thomas, slaves (servi) had the right to food, sleep, marriage, and the rearing of their children. Provision had also to be made for them to fulfill their religious duties, and they were to be treated with benevolence. With the conquest of the New World and the enslavement of whole populations of Indians and Africans, theologians such as Bartolomé de Las Casas and Cajetan began to object to the injustices of subjecting conquered peoples and of engaging in the lucrative slave trade. Some prominent Catholics of the early nineteenth century, including J.M. Sailer, Daniel O’Connell, and the Comte de Montalembert, together with many Protestants, pressed for the total abolition of slavery.
Throughout this period the popes were far from silent. As soon as the enslavement of native populations by European colonists started, they began to protest, although Noonan gives only a few isolated examples. Eugene IV in 1435 condemned the enslavement of the peoples of the newly colonized Canary Islands and, under pain of excommunication, ordered all such slaves to be immediately set free. Pius II and Sixtus IV emphatically repeated these prohibitions. In a bull addressed to all the faithful of the Christian world Paul III in 1537 condemned the enslavement of Indians in North and South America. Gregory XIV in 1591 ordered the freeing of all the Filipino slaves held by Spaniards. Urban VIII in 1639 issued a bull applying the principles of Paul III to Portuguese colonies in South America and requiring the liberation of all Indian slaves.
In 1781 Benedict XIV renewed the call of previous popes to free the Indian slaves of South America. Thus it was no break with previous teaching when Gregory XVI in 1839 issued a general condemnation of the enslavement of Indians and Blacks. In particular, he condemned the importation of Negro slaves from Africa. Leo XIII followed along the path set by Gregory XVI.
Although the popes condemned the enslavement of innocent populations and the iniquitous slave trade, they did not teach that all slaves everywhere should immediately be emancipated. At the time of the Civil War, very few Catholics in the United States felt that papal teaching required them to become abolitionists.
Bishop John England stood with the tradition in holding that there could be just titles to slavery. Bishop Francis P. Kenrick held that slavery did not necessarily violate the natural law. Archbishop John Hughes contended that slavery was an evil but not an absolute evil. Orestes Brownson, while denying that slavery was malum in se, came around to favor emancipation as a matter of policy.
In 1863 John Henry Newman penned some fascinating reflections on slavery. A fellow Catholic, William T. Allies, asked him to comment on a lecture he was planning to give, asserting that slavery was intrinsically evil. Newman replied that, although he would like to see slavery eliminated, he could not go so far as to condemn it as intrinsically evil. For if it were, St. Paul would have had to order Philemon, “liberate all your slaves at once.” Newman, as I see it, stood with the whole Catholic tradition. In 1866 the Holy Office, in response to an inquiry from Africa, ruled that although slavery (servitus) was undesirable, it was not per se opposed to natural or divine law. This ruling pertained to the kind of servitude that was customary in certain parts of Africa at the time.
No Father or Doctor of the Church, so far as I can judge, was an unqualified abolitionist. No pope or council ever made a sweeping condemnation of slavery as such. But they constantly sought to alleviate the evils of slavery and repeatedly denounced the mass enslavement of conquered populations and the infamous slave trade, thereby undermining slavery at its sources.
In A Church That Can and Cannot Change, Noonan gives only a few glimpses of this complex history. He correctly notes that the Catholic magisterium in past centuries never made an absolute condemnation of slavery as such. But he contends that John Paul II reversed the traditional teaching. In support he quotes a statement of John Paul II in 1992. Speaking at the infamous “House of Slaves” on the Island of Gorée in Senegal, from which innumerable slaves had been exported, he declared: “It is fitting to confess in all truth and humility this sin of man against man, this sin of man against God.” Noonan adds: “What this confession did not remark was how recently the sin had been discovered.” But if we look up the quotation, we will find that the pope is here speaking of the slave trade, which had repeatedly been condemned. Far from changing the doctrine, John Paul is explicitly reaffirming the position of Pope Pius II, whom he quotes as having declared in 1492 that the slave trade was an enormous crime, magnum scelus.’
Avery Cardinal Dulles, S.J., holds the Laurence J. McGinley Chair in Religion and Society at Fordham University.