Cappadocian Church Father, Gregory of Nyssa, lived in the 4th century and was an outspoken critic of slavery.
Because he was committed to the idea that humans have a unique value that demands respect, Gregory was an early and vocal opponent of slavery and also of poverty. Against the former Gregory marshals three arguments (Ecclesiastes IV ): (1) Only God has the right to enslave humans, and God does not choose to do so; indeed, it was God who gave human beings their free wills. (2) How dare a person take that precious entity–the only part of the created order to have been made in God’s image–and enslave it! (3) As humans who were created in the divine image, all people are radically equal; therefore, it is hubristic for some to arrogate to themselves absolute authority over others.
Robert B. Kruschwitzm from Baylor University writes:
The true offense of slavery, Gregory of Nyssa (c. 335-394) argued in his fourth homily on the book of Ecclesiastes, is that God created humans to be free. Commenting on the Teacher’s proud claim “I bought male and female slaves, and had slaves who were born in my house” (Ecclesiastes 2:7), Gregory wrote:
If man is in the likeness of God, and rules the whole earth, and has been granted authority over everything on earth from God, who is his buyer, tell me? Who is his seller? To God alone belongs this power; or rather, not even to God himself. For his gracious gifts, it says, are irrevocable (Romans 11:29). God would not there- fore reduce the human race to slavery, since he himself, when we had been enslaved to sin, spontaneously recalled us to freedom. But if God does not enslave what is free, who is he that sets his own power above God’s?