This is going to be a very complicated answer to say the least, especially when we see that the Apostle St. Paul himself never spoke out against slavery in the Apostolic Times.
St. Basil of Caesarea (329, bishopric 370-379) was the Greek bishop of Caesarea Kayseri in Turkey. He believed that all men are created equal by God and that no one is by nature a slave of another man. Despite this position, he accepted the condition of slavery which reigned in the society where he lived.
As early as the seventh century, Saint Bathilde (wife of King Clovis II) became famous for her campaign to stop slave-trading and free all slaves; in 851 Saint Anskar began his efforts to halt the Viking slave trade. That the Church willingly baptized slaves was claimed as proof that they had souls, and soon both kings and bishops—including William the Conqueror (1027-1087) and Saints Wulfstan (1009-1095) and Anselm (1033-1109)—forbade the enslavement of Christians.
Since, except for small settlements of Jews, and the Vikings in the north, everyone was at least nominally a Christian, that effectively abolished slavery in medieval Europe, except at the southern and eastern interfaces with Islam where both sides enslaved one another's prisoners. But even this was sometimes condemned: in the tenth century, bishops in Venice did public penance for past involvement in the Moorish slave trade and sought to prevent all Venetians from involvement in slavery. Then, in the thirteenth century, Saint Thomas Aquinas deduced that slavery was a sin, and a series of popes upheld his position, beginning in 1435 and culminating in three major pronouncements against slavery by Pope Paul III in 1537.
The Catholic Church has condemned the institution of slavery on several occasions.
Seeing that the Church has condemned slavery prior to the institution of the Atlantic Slavery Trade, it stands to reason that this also is to be considered condemned by Catholicism.
From 1435 to 1890, we have numerous bulls and encyclicals from several popes written to many bishops and the whole Christian faithful condemning both slavery and the slave trade. The very existence of these many papal teachings during this particular period of history is a strong indication that from the viewpoint of the Magisterium, there must have developed a moral problem of a different sort than any previously encountered. In this article I will address three—from many more—of the responses of the papal Magisterium to the widespread enslavement that accompanied the Age of Discovery and beyond.
Eugene IV: Sicut Dudum, 1435
On January 13, 1435, Eugene IV issued from Florence the bull . Sent to Bishop Ferdinand, located at Rubicon on the island of Lanzarote, this bull condemned the enslavement of the black natives of the newly colonized Canary Islands off the coast of Africa. The Pope stated that after being converted to the faith or promised baptism, many of the inhabitants were taken from their homes and enslaved:
"They have deprived the natives of their property or turned it to their own use, and have subjected some of the inhabitants of said islands to perpetual slavery (subdiderunt perpetuae servituti), sold them to other persons and committed other various illicit and evil deeds against them.... Therefore We ... exhort, through the sprinkling of the Blood of Jesus Christ shed for their sins, one and all, temporal princes, lords, captains, armed men, barons, soldiers, nobles, communities and all others of every kind among the Christian faithful of whatever state, grade or condition, that they themselves desist from the aforementioned deeds, cause those subject to them to desist from them, and restrain them rigorously. And no less do We order and command all and each of the faithful of each sex that, within the space of fifteen days of the publication of these letters in the place where they live, that they restore to their pristine liberty all and each person of either sex who were once residents of said Canary Islands ... who have been made subject to slavery (servituti subicere). These people are to be totally and perpetually free and are to be let go without the exaction or reception of any money."
Paul III: Sublimis Deus, 1537
The pontifical decree known as "The Sublime God" has indeed had an exalted role in the cause of social justice in the New World. Recently, authors such as Gustavo Gutierrez have noted this fact: 'The bull of Pope Paul III, Sublimis Deus (June 2, 1537), is regarded as the most important papal pronouncement on the human condition of the Indians." It is, moreover, addressed to all of the Christian faithful in the world, and not to a particular bishop in one area, thereby not limiting its significance, but universalizing it.
Sublimis Deus was intended to be issued as the central pedagogical work against slavery. Two other bulls would be published to implement the teaching of Sublimis, one to impose penalties on those who fail to abide by the teaching against slavery, and a second to specify the sacramental consequences of the teaching that the Indians are true men.
The first central teaching of this beautiful work is the universality of the call to receive the Faith and salvation:
"And since mankind, according to the witness of Sacred Scripture, was created for eternal life and happiness, and since no one is able to attain this eternal life and happiness except through faith in our Lord Jesus Christ, it is necessary to confess that man is of such a nature and condition that he is capable to receive faith in Christ and that everyone who possesses human nature is apt for receiving such faith . . . Therefore the Truth Himself Who can neither deceive nor be deceived, when He destined the preachers of the faith to the office of preaching, is known to have said: 'Going, make disciples of all nations.' 'All,' he said, without any exception, since all are capable of the discipline of the faith."
The teaching of Sublimis continued:
"Seeing this and envying it, the enemy of the human race, who always opposes all good men so that the race may perish, has thought up a way, unheard of before now, by which he might impede the saving word of God from being preached to the nations. He has stirred up some of his allies who, desiring to satisfy their own avarice, are presuming to assert far and wide that the Indians of the West and the South who have come to our notice in these times be reduced to our service like brute animals, under the pretext that they are lacking the Catholic Faith. And they reduce them to slavery (Et eos in servitutem redigunt), treating them with afflictions they would scarcely use with brute animals."
Gregory XVI: In Supremo, 1839
The 1839 Constitution In Supremo by Gregory XVI continued the antislavery teaching of his predecessors, and was in the same manner not accepted by many of those bishops, priests and laity for whom it was written. As we will see, even today many authors do not have an accurate understanding of this work. First, however, let us consider the content of In Supremo itself.
The introduction of In Supremo tells us that it was written to turn Christians away from the practice of enslaving blacks and other peoples. In it, Gregory first mentioned the efforts of the Apostles and other early Christians to alleviate out of the motive of Christian charity the suffering of those held in servitude, and that they encouraged the practice of emancipating deserving slaves. At the same time, he noted that:
"There were to be found subsequently among the faithful some who, shamefully blinded by the desire of sordid gain, in lonely and distant countries did not hesitate to reduce to slavery (in servitutem redigere) Indians, Blacks and other unfortunate peoples, or else, by instituting or expanding the trade in those who had been made slaves by others, aided the crime of others. Certainly many Roman Pontiffs of glorious memory, Our Predecessors, did not fail, according to the duties of their office, to blame severely this way of acting as dangerous for the spiritual welfare of those who did such things and a shame to the Christian name."
Gregory then cited the various predecessors and their antislavery teachings, even recalling the familiar phrase in contained in the work of Paul III and his successors. He mentioned the efforts of Clement I, Pius II, Paul III, Benedict XIV, Urban VIII and Pius VII, before concluding this historical summary:
"Indeed these sanctions and this concern of Our Predecessors availed in no small measure, with the help of God, to protect the Indians and the other peoples mentioned from the cruelties of the invaders and from the greed of Christian traders."
However Gregory was well aware that there was still much work to be done:
"The slave trade, although it has been somewhat diminished, is still carried on by numerous Christians. Therefore, desiring to remove such a great shame from all Christian peoples ... and walking in the footsteps of Our Predecessors, We, by apostolic authority, warn and strongly exhort in the Lord faithful Christians of every condition that no one in the future dare to bother unjustly, despoil of their possessions, or reduce to slavery () Indians, Blacks or other such peoples. Nor are they to lend aid and favor to those who give themselves up to these practices, or exercise that inhuman traffic by which the Blacks, as if they were not humans but rather mere animals, having been brought into slavery in no matter what way, are, without any distinction and contrary to the rights of justice and humanity, bought, sold and sometimes given over to the hardest labor."
For the early 19th century, in the midst of the volatile decades before the Civil War, Gregory XVI issued In Supremo, with its clear condemnation of both the slave trade and slavery itself.
Since that Constitution mentioned the documents of the previous pontiffs, it is hard to understand how the American hierarchy was not aware of the consistency of the teaching and its nature.
All of these teachings, nonetheless, went unknown to the Catholic faithful of the U.S., perhaps through willful ignorance, or were explained away by many of the American bishops and clergy. Thus, we can look to the practice of dissent from the teachings of the Papal Magisterium as a key reason why slavery was not directly opposed by the Church in the United States.
In the light of Humanae Vitae of Pope Paul VI, and Veritatis Splendor and Evangelium Vitae of John Paul II, can we not hope that the shepherds of the Church will not fall into the same mistakes of their predecessors?
The Popes and Slavery: Setting the Record Straight