Slavery was as common to early Christians as the grass that grows in the field. It was a part of everyday life. The early church was not at all concerned with getting rid of slavery from the societies they lived in. There are a number of verses in the New Testament that tell slaves to obey their masters, not run away from them or fight against them. If one can get their freedom though, that is not a bad thing.
Men like Paul probably understood that enslaving other humans beings is not a good thing, but the Kingdom of Heaven is not about flesh and blood. The Kingdom of Heaven is concerned with much more important matters than if someone is a slave to another person or not. It changes people from the inside out. If you free a slave from his earthly master, he may then be a free man. But that free man is still a slave to sin in his heart, unless someone sets him free from that. Jesus set's men's hearts free. Whether or not they are physical slaves to other men or not is not even really a secondary issue, it's farther down the line than that.
In first century Rome, being a slave wasn't always a terrible thing. There were laws that required masters to feed and house their slaves. Many "Free Men" died of starvation when they couldn't find work, while slaves across the street remained alive. Plus, a good slave could be adopted into the family he or she served, and would then have all the rights a son or daughter would have and inheritance. Actually mores than that, an adopted slave could not be disowned by the family or cut out of the inheritance, whereas a natural son could be.
Slavery is wrong. But it's not the biggest issue there is by any means. When we make it into a bigger issue than the Gospel of Jesus Christ, we distract people from the truth and do more harm.
I'm glad we don't have slavery in our modern society like we used to. But, most men and women I run into are still slaves to sin in their hearts. Those who are so concerned with social justice issues have no power to set these slaves free. Jesus does though, through his glorious gospel.
Grace to you,
Travis