Here is a short summary of the reasons American Christians before the Civil War believed slavery was biblical. They used Bible verses. They used Old Testament and New Testament verses.
Recall the previous letter, part 6. In Acts we see the church retaining a few Old Testament rules but shortly thereafter, dropping even those. Paul says to the church in Corinth, "look, if you want to eat food sacrificed to idols, fine, just don't flaunt it." As far as Paul is concerned, the loving your neighbor law is the one that matters. Otherwise, food is food. We see this principle in action - the church continues to figure out what loving our neighbors looks like.
After the apostles died, some people in the church realized that owning a slave is not the way one would love oneself, thus not fulfilling Christ's Golden Rule. This took centuries to gain widespread acceptance. Shameful, I know. Even great revivalist theologians in America did not see slavery that way. For example Jonathan Edwards owned an African slave and George Whitefield not only owned slaves, while leading the First Great Awakening, he successfully argued against Georgia's anti-slavery laws. As you know, America could not settle this topic theologically, but by an horrific civil war. the south was deeply Christian, yet it seems they were more biblical than Christian.
The early church did not oppose the culture of slavery, but accommodated it. Most of the early church's converts were poor, even slaves themselves, yet slave owners were also welcomed into the fellowship. The cultural imagination of the church was limited. Eventually, the Holy Spirit opened the eyes of individuals who began to champion the idea that slavery, though Biblical, did not conform to Christ's law of love.
Here is the cultural (Biblical) imagination the church eventually abandoned, as reflected in Exodus 21.
20 Anyone who beats their male or female slave with a rod must be punished if the slave dies as a direct result, 21 but they are not to be punished if the slave recovers after a day or two, since the slave is their property.That is an unhappy story. Yet it serves as part of Jesus' story. It's the culture he comes from and he challenges this culture by fulfilling it. He considers himself a slave. He tells his disciples that the greatest in his kingdom is the slave to everyone else. Jesus's kingdom is not based on power, but on love expressed through submission and service. Jesus acted as a house slave when he washed his disciples' feet at their Last Supper before his crucifixion. Jesus honors the slave by acting the slave. It is our error to take advantage of slaves, instead of serving them, and if we are to serve them how can we own them. Jesus famously says, whatever we do to the least of these, we do to him. How can we beat him? Jesus is incarnate in the least among us. This is the Christ-like approach to the world.
This is a good story. It is good news. If you want to argue biblically against slavery, you will lose. If you want to argue from Jesus' law of love, you win, because that single proof text of the Golden Rule cannot be overruled. It overrules every other one.
As imperfect human beings, growing into maturity as followers of Jesus, we get this wrong. We don't serve everyone. We do take advantage of the least of these, not to the extent of those before us. George Whitefield and Jonathan Edwards got it really wrong. You will as well. Hopefully, you will stop appealing to Biblical excuses to act like an anti-Christ. Please understand, not everything Biblical is Christian.
Part one points out that the words of Satan recorded in the Bible are not Christian doctrine. Part two shows the Sermon on the Mount overruling the cursing of enemies exhibited in Psalm 137. Parts three and four show Moses getting overruled by Ezekiel and Jesus. Part five merely brushes the concept of source criticism. Part six looks at the Old Testament application in the early church: a brief summary of the book of Acts.