Hilaire Belloc enlightened me to the meaning of pre-christian slavery in the Servile State.
There was no question in those ancient societies from which we spring of making subject races into slaves by the might of conquering races. All that is the guess-work of the universities. Not only is there no proof of it, rather all the existing proof is the other way. The Greek had a Greek slave, the Latin a Latin slave, the German a German slave, the Celt a Celtic slave.
The theory that "superior races" invaded a land, either drove out the original inhabitants or re-duced them to slavery, is one which has no argument either from our present knowledge of man's mind or from recorded evidence. Indeed, the most striking
feature of that Servile Basis upon which Paganism reposed was the human equality recognised between master and slave. The master might kill the slave, but both were of one race and each was human to the other.
You might say, well that's well and good, but that's pre-christian slavery. What about Christian slavery? Well, 1.) that's a misnomer and 2.) the dark ages were a slow process of forgetting slavery through private or collective ownership by the people. It may not have been the intended effect, but the crusades did more to free English serfs (who by that time were technically free from slavery, but tied to the land which they did not own) than any other movement of the last 2000 years. And if you can't call the Crusades a product of Christianity, I'm not sure what you can call a product of Christianity.
During the Crusades, a noble would have to finance his journey to the Holy Land by granting large tracts of land to the serfs whom he previously lorded over. Sometimes, in exchange for not granting these tracts of land, they'd just ask for more freedoms for the serfs1.
That's all tangential to the point in the question, but I mention it to draw the distinction between what effect the Gospel message had on slavery. People became willing to sell all they had (slaves, land and other possessions) just to do what they thought was God's will.
Philemon for instance, is a tacit acceptance of slavery as it was. Part of the epistle is read at Mass once every 3 years and it is almost always followed by a homily about race relations, civil rights, etc... What should be carried away is that in Christ we're all brothers and sisters.
So, the point is, if Jesus doesn't call us slaves, he calls us friends. And in ancient times, slavery wasn't about a master race enslaving all the poor races of the world (even Saul/Paul was granted the rights as a Roman citizen as a Jew). Slaves are those individuals who are bound to their master, there is no slave race bound to a master race. Being a servant or a slave, to Jesus and Paul and Jude, was a very, very good thing.
1. I known I'm going to get beat down by an Englishman for pretending to know anything about medieval England, so I'll just say this is my synthesis of Belloc and Chesterton's histories and if you haven't read them then you can't really call yourself an Englishman anymore than I can!