Toward the end of the fifteenth century, at which time the base stain of slavery having been nearly blotted out from among Christian nations, States were anxious to stand firmly in evangelical liberty, and also to increase their empire, this apostolic see took the greatest care that the evil germs of such depravity should nowhere revive. She therefore directed her provident vigilance to the newly discovered regions of Africa, Asia, and America; for a report had reached her that the leaders of those expeditions, Christians though they were, were wickedly snaking use of their arms and ingenuity for establishing and imposing slavery on these innocent nations. Indeed, since the crude nature of the soil which they had to overcome, nor less the wealth of metals which had to be extracted by digging, required very hard work, unjust and inhuman plans were entered into. For a certain traffic was begun, slaves being transported for that purpose from Ethiopia, which, at that time, under the name of La tratta dei Negri, too much occupied those colonies. An oppression of the indigenous inhabitants (who are collectively called Indians), much the same as slavery, followed with a like maltreatment.
When Pius II had become assured of these matters without delay, on October 7, 1462, he gave a letter to the bishop of the place in which he reproved and condemned such wickedness. Some time afterwards, Leo X lent, as far as he could, his good offices and authority to the kings of both Portugal and Spain, who took care to radically extirpate that abuse, opposed alike to religion, humanity, and justice. Nevertheless, that evil having grown strong, remained there, its impure cause, the unquenchable desire of gain, remaining. Then Paul III, anxious with a fatherly love as to the condition of the Indians and of the Moorish slaves, came to this last determination, that in open day, and, as it were, in the sight of all nations, he declared that they all had a just and natural right of a threefold character, namely, that each one of them was master of his own person, that they could live together under their own laws, and that they could acquire and hold property for themselves. More than this, having sent letters to the Cardinal Archbishop of Toledo, he prounounced an interdict and deprival of sacraments against those who acted contrary to the aforesaid decree, reserving to the Roman Pontiff the power of absolving them.
With the same forethought and constancy, other Pontiffs at a later period, as Urban VIII, Benedict XIV, and Pius VII, showed themselves strong asserters of liberty for the Indians and Moors and those who were even as yet not instructed in the Christian faith. The last, moreover, at the Council of the confederated Princes of Europe, held at Vienna, called their attention in common to this point, that that traffic in Negroes, of which We have spoken before, and which had now ceased in many places, should be thoroughly rooted out. Gregory XVI also severely censured those neglecting the duties of humanity and the laws, and restored the decrees and statutory penalties of the apostolic see, and left no means untried that foreign nations, also, following the kindliness of the Europeans, should cease from and abhor the disgrace and brutality of slavery. But it has turned out most fortunately for Us that We have received the congratulations of the chief princes and rulers of public affairs for having obtained, thanks to Our constant pleadings, some satisfaction for the long-continued and most just complaints of nature and religion. —from In Plurimus (1888)