Calvin on the Lawfulness of Slavery

“Here a question arises: Is perpetual servitude so displeasing to God, that it ought not to be deemed lawful? To this the answer is easy — Abraham and other fathers had servants or slaves according to the common and prevailing custom, and it was not deemed wrong in them. Before the Law was given, there was nothing to forbid one who had servants or maids to exercise power over them through life; and then the Law, mentioned here, was not given indiscriminately and generally, but it was a peculiar privilege in favor of the chosen people. Hence it is without reason that any one infers that it is not lawful to exercise power over servants and maids; for, on the contrary, we may reason thus, That since God permitted the fathers to retain servants and maids, it is a thing lawful; and further, as God permitted the Jews also, under the Law, to bear rule over aliens, and to keep them perpetually as servants, it follows that this cannot be disapproved. And still a clearer evidence may be adduced; for since the Gentiles have been called to the hope of salvation, no change has in this respect been made. For the Apostles did not constrain masters to liberate their servants, but only exhorted them to use kindness towards them, and to treat them humanely as their fellow-servants. (Ephesians 6:9; Colossians 4:1) If, then, servitude were unlawful, the Apostles would have never tolerated it; but they would have boldly denounced such a profane practice had it been so. Now, as they commanded masters only to be humane towards their servants, and not to treat them violently and reproachfully, it follows that what was not denied was permitted, that is, to retain their own servants. We also see that Paul sent back Onesimus to Philemon. (Philemon 1:12) Philemon was not only one of the faithful, but a pastor of the Church. He ought, then, to have been an example to others. His servant had fled away from him; Paul sent him back, and commended him to his master, and besought his master to forgive his theft. We hence see that the thing in itself is not unlawful.”

— John Calvin, Commentary on Jeremiah 34:8-17