What was the biblical basis for the abolition movement? As I mentioned before, Genesis 1:27 should be the basis for all our dealings with other people in any situation. The abolitionists (who were mostly fervent Christians) argued (and of course I agree) that any system that degrades and despises the image of God (which is found in every human being) is--right off the bat--incompatible with Scripture.
The Bible is certainly not racist, so any attempt to use it to justify the race-based slavery in the U.S. should've been immediately suspect. The belief that one pigment of skin is superior to another finds absolutely no basis in Scripture. The slavery mentioned in the Bible was not based on racism; in fact, most of the slaves were prisoners of war or who'd sold themselves in order to pay a debt.
While it is true that Paul didn’t condemn slavery as intrinsically sinful and didn't call for its immediate abolition, I believe that he attacked it “from the inside-out.” The notion that a master is of more “worth” than a slave is contradicted by today’s passage: As commonly noted, the ground is all level at the foot of the cross. Ephesians 6:5-9, in its instructions for masters, taught that God holds no “double standard” and will judge all of us by the same yardstick. The strongest piece of evidence, however, is the entire book of Philemon, where Paul urged reconciliation between a fugitive slave and his former master. He openly encouraged Philemon (the owner) to see his slave as a dear brother (vs. 15-16), not a piece of property. Finally, Paul deliberately equated himself with the fugitive slave: After reminding him of Philemon’s debt to Paul (for leading him to Christ), he wrote these startling words in verse 17: “So if you consider me a partner, welcome him as you would welcome me” (emphasis mine). Thus Paul the apostle equated himself with the lowest person on the "totem pole," an escaped slave.
We need to remember that the Bible’s primary focus is our relationship with God, which first and foremost affects our relationship with other individuals. Social and legal reform is important, but Paul’s approach towards slavery is not immediate abolition but the undermining of it through adjusting our view of other people (as image-bearers and siblings in Christ). As Christians changed their outlook, finally they started asking themselves, “How can I claim to own my own brother, like an animal or a piece of furniture?” The laws followed suit.
Finally, the two best biblical arguments I have heard against slavery were most famously put forward by Abraham Lincoln himself, even though he wasn't exactly a biblical scholar. First, as Thomas Krannawitter points out, Lincoln argued that human equality "is the father of all moral principle." As I mentioned earlier, the fact that we're all created in God's image confers a worth/value on us and obviously refutes any system based on any type of slavery, especially race-based like America's.
Second, he actually applied the "Golden Rule": If I wouldn’t want to be a slave, I shouldn’t own slaves.
Of course, no one is really defending slavery today, but the Bible speaks of an even worse form of bondage. Jesus told us in John 8:34 that “everyone who sins is a slave to sin,” but two verses later he also promised that “if the Son sets you free, you will be free indeed.” I might not have physical chains around my neck, but outside of Christ all of humanity is trapped by a Master who's far more cruel and oppressive than any plantation owner. Thank the Lord Jesus, we who belong to him are not slaves but heirs of God and co-heirs with Christ.
Lord Jesus, you have freed me from my chains. I want to live in your freedom. Please help me.