When we think of slavery it is the experience of blacks in America which most often comes to mind. Africans targeted by commercial slave traders as ignorant savages were captured in raids, transported across the Atlantic and sold on the open market like livestock.
In the Roman Empire slavery was not tied to race as it was here. The first slaves seem to have been children sold by their own parents and enemy warriors and their families captured during battle. Debtors sold themselves into slavery to cover their debts. They could be freed if their family or friends paid off the debt or even as a reward for exceptional service. They might then enjoy the patronage of an employer, their former master or someone else recognized in a public ceremony.
This last example seems to have been the model of slavery St. Paul had in mind when he wrote of people set free from sin and become “slaves of righteousness” (Romans 6:16) through baptism. Incorporation into Christ was seen as delivering people from slavery and connecting them to a new Protector whom they would now serve.