14 There are also many other good deeds of the Church in the same behalf. For she, indeed, was accustomed by severe penalties to defend slaves from the savage anger and cruel injuries of their masters. To those upon whom the hand of violence had rested, she was accustomed to open her sacred temples as places of refuge to receive the free men into her good faith, and to restrain those by censure who dared by evil inducements to lead a man back again into slavery. In the same way she was still more favorable to the freedom of the slaves whom, by any means she held as her own, according to times and places; when she laid down either that those should be released by the bishops from every bond of slavery who had shown themselves during a certain time of trial of praiseworthy honesty of life, or when she easily permitted the bishops of their own will to declare those belonging to them free. It must also be ascribed to the compassion and virtue of the Church that somewhat of the pressure of civil law upon slaves was remitted, and, as far as it was brought about, that the milder alleviations of Gregory the Great, having been incorporated in the written law of nations, became of force. That, however, was done principally by the agency of Charlemagne, who included them in his Capitularia, as Gratian afterwards did in his Decretum.(23) Finally, monuments, laws, institutions, through a continuous series of ages, teach and splendidly demonstrate the great love of the Church toward slaves, whose miserable condition she never left destitute of protection, and always to the best of her power alleviated. Therefore, sufficient praise or thanks can never be returned to the Catholic Church, the banisher of slavery and causer of true liberty, fraternity, and equality among men, since she has merited it by the prosperity of nations, through the very great beneficence of Christ our Redeemer.
Source: In Plurimis (Vatican.va)