12 The care of the Church extended to the protection of slaves, and without interruption tended carefully to one object, that they should finally be restored to freedom, which would greatly conduce to their eternal welfare. That the event happily responded to these efforts, the annals of sacred antiquity afford abundant proof. Noble matrons, rendered illustrious by the praises of St. Jerome, themselves afforded great aid in carrying this matter into effect; so that as Salvian relates, in Christian families, even though not very rich, it often happened that the slaves were freed by a generous manumission. But, also, St. Clement long before praised that excellent work of charity by which some Christians became slaves, by an exchange of persons, because they could in no other way liberate those who were in bondage. Wherefore, in addition to the fact that the act of manumission began to take place in churches as an act of piety, the Church ordered it to be proposed to the faithful when about to make their wills, as a work very pleasing to God and of great merit and value with Him. Therefore, those precepts of manumission to the heir were introduced with the words, "for the love of God, for the welfare or benefit of my soul."(22) Neither was anything grudged as the price of the captives, gifts dedicated to God were sold, consecrated gold and silver melted down, the ornaments and gifts of the basilicas alienated, as, indeed, was done more than once by Ambrose, Augustine, Hilary, Eligius, Patrick, and many other holy men.
Source: In Plurimis (Vatican.va)