But that was totally, utterly ignored by the rest of the world:
In 1839, Gregory wrote that:
[W]e have judged that it belonged to Our pastoral solicitude to exert Ourselves to turn away the Faithful from the inhuman slave trade in Negroes and all other men. [...] [D]esiring to remove such a shame from all the Christian nations, having fully reflected over the whole question and having taken the advice of many of Our Venerable Brothers the Cardinals of the Holy Roman Church, and walking in the footsteps of Our Predecessors, We warn and adjure earnestly in the Lord faithful Christians of every condition that no one in the future dare to vex anyone, despoil him of his possessions, reduce to servitude, or lend aid and favour to those who give themselves up to these practices, or exercise that inhuman traffic by which the Blacks, as if they were not men but rather animals, having been brought into servitude, in no matter what way, are, without any distinction, in contempt of the rights of justice and humanity, bought, sold, and devoted sometimes to the hardest labour. Further, in the hope of gain, propositions of purchase being made to the first owners of the Blacks, dissensions and almost perpetual conflicts are aroused in these regions. We reprove, then, by virtue of Our Apostolic Authority, all the practices above mentioned as absolutely unworthy of the Christian name. By the same Authority We prohibit and strictly forbid any Ecclesiastic or lay person from presuming to defend as permissible this traffic in Blacks under no matter what pretext or excuse, or from publishing or teaching in any manner whatsoever, in public or privately, opinions contrary to what We have set forth in this Apostolic Letter.
The Roman Catholic Church, in 1839, came out squarely against slavery.
Can you even imagine if the world listened and followed?