It is a sin against the dignity of persons and their fundamental rights to reduce them by violence to their productive value or to a source of profit. St. Paul directed a Christian master to treat his Christian slave, “Not now as a servant [slave], but above a servant, a brother beloved ... both in the flesh and in the Lord” [Philemon]? Paul’s letter to Philemon, sometimes in the past used to justify slavery, was now given paradigmatic force against slavery. The [7th] commandment against theft, once used by theologians to forbid a slave to escape, was now treated as the most relevant commandment against slavery ... Awkwardness was hardly avoidable when support for the condemnation of enslavement was sought among commandments two of which had given divine approval to the institution.
The Church That Can and Cannot Change, p. 121