Super short version, post-Constantinian Church Fathers did start speaking against slavery pretty quickly either to condemn it outright or say that though it was a social evil it was one that had to be tolerated though not approved of. Gregory the Theologian, bishop of Nazianzus, spoke against slavery in the 4th century CE. Orthoduck briefly notes that for those who think Jesus and the Church have against them that they did not condemn slavery just haven't read enough patristics yet. In a post Constantinian setting where the Church Fathers didn't have to wonder if they were going to get reamed for speaking up against the institution Church Fathers started condemning owning people as property. Perhaps some people didn't notice the part where "kidnappers" or "slaver traders" were condemned in the NT documents?
Anyway, this can be considered the monthly semi-scheduled link to Orthocuban now that I'm thinking of it. :)