13 Moreover, the Roman Pontiffs, who have always acted, as history truly relates, as the protectors of the weak and helpers of the oppressed, have done their best for slaves. St. Gregory himself set at liberty as many as possible, and in the Roman Council of 597 desired those to receive their freedom who were anxious to enter the monastic state. Hadrian I maintained that slaves could freely enter into matrimony even without their masters' consent. It was clearly ordered by Alexander III in the year 1167 to the Moorish King of Valencia that he should not make a slave of any Christian, because no one was a slave by the law of nature, all men having been made free by God. Innocent III, in the year 1190, at the prayer of its founders, John de Matha and Felix of Valois, approved and established the Order of the Most Holy Trinity for Redeeming Christians who had fallen into the power of the Turks. At a later date, Honorius III, and, afterwards, Gregory IX, duly approved the Order of St. Mary of Help, founded for a similar purpose, which Peter Nolasco had established, and which included the severe rule that its religious should give themselves up as slaves in the place of Christians taken captive by tyrants, if it should be necessary in order to redeem them. The same St. Gregory passed a decree, which was a far greater support of liberty, that it was unlawful to sell slaves to the Church, and he further added an exhortation to the faithful that, as a punishment for their faults, they should give their slaves to God and His saints as an act of expiation.
Source: In Plurimis (Vatican.va)