Council of Gangra: “If anyone, on the pretext of religion, teaches another man’s slave to despise his master and to withdraw from his service, and not serve his master with good will and all respect, let him be anathema.”
1452: Pope Nicholas V wrote Dum Diversas which granted to the kings of Spain and Portugal the right to reduce any “Saracens [Muslims] and pagans and any other unbelievers” to perpetual slavery.
Pope Urban VIII in 1629, Pope Innocent X in 1645 and Pope Alexander VII in 1661 were all personally involved in the purchase of Muslim slaves.
As there were also popes who had concubines and illegitimate children would you take this to mean that their behavior was also Church doctrine? I think your stronger arguments are Dum Diversas and the pronouncements of the Council of Gangra.
Regarding them both, it is not at all clear that our use of the term slave has the same meaning it did for them. For example, this comment from Aquinas:
“A man becomes another’s slave not only by being overcome by him, but also by subjecting himself to him spontaneously.” (ST, I/II 80, 4 ad 2) The “subjecting himself spontaneously” part seems a good bit like indentured servitude. The connection between the terms is even more apparent in this quote where he uses servants and slaves as synonyms.
"With regard to the ill-treatment of servants, the Law seems to have taken into consideration whether it was certain or not: since if it were certain, the Law fixed a penalty: for maiming, the penalty was forfeiture of the servant, who was ordered to be given his liberty: while for slaying, the punishment was that of a murderer, when the slave died under the blow of his master." (ST I/II 105, 4 ad 3)
Regarding the Council of Gangra, their comment would surely be reasonable according to the view that slave=indentured servant, as Aquinas noted:* “…and inducing a slave to leave his master are properly injuries against the person; yet the latter, since a slave is his master’s chattel, is referred to theft.”* (ST II/II 61,3)
You also might consider the reason that council, which took place in the middle of the fourth century, even felt it necessary to make such a proclamation. It is reasonable to consider the strong likelihood that, as Christianity was becoming the religion of the Empire, there were some who were holding that this meant all slaves - bonded or otherwise - were to be freed. Opposing that notion is not the same as supporting slavery.
Ender