Two months ago I read an intriguing contribution of Cynthia Nielsen at her blog Per Caritatem, titled: Augustine and Scotus on Slavery. Since then I have been thinking about this question: what did Calvin say about Slavery? I 
did some research, but didn’t come very far. However, very recently I laid my hands upon a new book about Calvin. (Unfortunately for some of you, it’s for the moment only available in Dutch: Johannes Calvijn – zijn leven, zijn werk (edited by: Willem Balke, Jan C. Klok, Willem van ‘t Spijker)). In this book prof. Willem Balke writes about Calvin and Slavery. It is a very instructive entry. Let me summarize Calvin’s thoughts:
1. Slavery is, according to Calvin, against God’s ordination, since God created mankind in freedom. Freedom is, so to speak, man’s natural condition. And slavery is the denial of freedom. Slavery therefore is an unnatural condition. In this respect Calvin’s position is quite like Augstine’s.
2. This freedom has been lost for a great part of humanity, ‘shortly after the deluge’ (CO 23,179). About the cause of this loss is Calvin explicit: the cause is sin. Historically spoken, he is presuming that the moment that war came into existence, slavery was also introduced, since in a war enemies are usually treated as slaves. But human beings are created as brothers, ment for maintaining mutual fellowship (CO 23,179).
3. But what about the legislation in the Old Testament about slavery? Calvin points out that these laws are ‘provisional’, but that God’s intention always was to set people free. The laws about slaves and slavery are no authentication of slavery, but a curtailment of the evil of this institution. Without these laws the situation would have been far worse, since ‘human beings can’t get used to be charitable to their fellow men’ (CO 27,344).
4. Calvin stresses the fact that the Old Testament laws not only require the periodic deliverance of slaves, but also prescribe what is necessary to take possession of that freedom.
“God says clearly that one should help those who leave the house of their master. Why? Because when someone is send away naked, he will choose to stay in slavery instead of leaving and being free.” (CO 27,345)
In other words, his ethical recommendations aim at the restoration of man’s freedom as creature and child of God.
Qualitative Theology 