17 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.(25) 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.
Source: In Plurimis (Vatican.va)