1. Slavery in the first century was quite different from slavery in early American history. For one thing, Roman slaves were either taken as the spoils of war or were such because they sold themselves into slavery (known as "bond-servant"). They were often well-educated (cf. Gal 3:24 in which the "tutor" or better "disciplinarian" or "guide" of the children was usually a slave). The normal word for "slave" in the New Testament is the term dou'lo", a term that in earlier centuries usually referred to one who sold himself into slavery; later on, it was used especially of those who became slaves as the spoils of war.
2. Although the masters had absolute rights over their slaves, they generally showed them respect, very unlike the South in the days of Lincoln. They often treated them with human dignity and, although they could beat them, such does not seem to be as regular a practice as it was in America. Slaves could marry, accumulate wealth, purchase their own freedom, run a business, etc. Cicero noted that a slave could usually be set free within seven years; in any case, under Roman law a slave would normally be set free by age 30. All this can be overstated, however. The revolt led by Spartacus in 73 BC caused Rome to treat slaves from the western regions more harshly (very similar to how black slaves were treated). Eastern slaves, however, enjoyed much greater freedom.
3. As much as two thirds of the Roman empire were slaves (before the first century it was as high as 90%). By the first century AD an increasingly large number of slaves were being freed?so much so that Caesar had to write up laws that governed the procedure! Quite different from the Old South where only South Carolina had more slaves than freemen (so far as I know).
One of the implications of this has to do with the NT authors' strategy on slavery: Should Paul tell the slaves to rebel? Could he write an emancipation proclamation? When we think through this issue, it is plain that the NT writers simply could not outright condemn slavery (the disastrous results of Spartacus' rebellion [in spite of the Hollywood portrayal] would have been etched in their minds). Further, to whom would such a directive be pointed? To the pagan masters? They do not place themselves under God's law and are not a part of his kingdom program. Paul's exhortations to them would be meaningless. To the slaves? They are powerless to bring about their own freedom apart from overt actions (e.g., rebellion, running away). Further, such actions hardly comported with the gospel: change is to take place from the inside out, not from imposition on social structures. (The one exception to this had to do with ultimate allegiance and worship: civil disobedience was always encouraged when it came to having to choose between Christ and Caesar.)
4. There are passages in Paul that speak to the issue of slavery, and they at least plant the seeds of freedom as concomitant with the gospel. The entire letter to Philemon addresses the issue of Onesimus' freedom. Paul urges Philemon to free him because Onesimus had become useful to Paul in ministry. Paul does not command Philemon; he urges. He even says that he could command him, as his father in the faith, but he wants Philemon to make the decision from his heart. Herein lies the point at stake: Is the NT about social change first or is it about change of the heart? As much as I believe that Christians should become involved in several aspects of society (we are, after all, "the salt of the world"), when we exchange the gospel for a merely social agenda we contaminate our mission. I believe there are social implications of the gospel that are quite extensive, but let us never forget that our primary task in relation to the world is not to change political structures, but to offer forgiveness of sin in the name of Jesus Christ.
In Eph 6:5-9 Paul reminds masters of their responsibility to treat their slaves with respect, noting that they too have a Master in heaven.
5. Now, with this background in mind, let's look briefly at a couple of passages: Col 3:22-25 and 1 Cor 7:17-24. The first text only gives instructions to those who are slaves to perform their duties well. I think that if Paul lived in Dixie 150 years ago, his advice to Christian slaves would be the same. His advice to Christian masters would be quite different: he would ask them to treat their slaves with dignity and respect and hope that they would come to recognize the incompatibility of slavery with the gospel. He could certainly write on this topic, too. But Paul would not tell the slaves to rebel or run away. He always sought change from the inside.
As for 1 Cor 7:17-24: In v. 21 the NASB reads: "Were you called while a slave? Do not worry about it; but if you are able also to become free, rather do that." The NIV and REB are similar. The NRSV takes this verse to mean "Were you a slave when called? Do not be concerned about it. Even if you can gain your freedom, make use of your present condition now more than ever." The last two words in Greek are in dispute and scholars are divided on them. That's because the adverb, ma'llon, can mean "rather" or "all the more." Is Paul saying that they should not seek their freedom even if they can, or is he saying that by all means take it if it comes to them? Difficult to tell, though I favor the view that the slaves were to take their freedom if it were offered to them. However, 1 Cor 7 is governed by v. 26?"the present necessity" or the "present crisis." Whatever that was, it seemed to have put a hold on normal life. After all, in this chapter it looks as though Paul is telling people that it's best not to get married. He has to explicitly refute that this is his normal teaching a few years later (1 Tim 4:1-3; 5:14). The crisis in the Corinthian church may have been one of persecution or a financial issue (for a famine had swept the Mediterranean region ten years earlier). In such unstable times, it's usually best to lie low until the crisis is past.
Finally, on 1 Cor 7, Paul does make one significant pronouncement: In v. 23 he says, "You were bought with a price; do not become slaves of men." Here we clearly see his attitude toward slavery and the seeds of social freedom embedded in his words. The gospel and slavery are incompatible because someone else has already purchased us.