What does the Bible say about slavery?
First, it's important to understand that the concept of slavery found in the Bible is very different than the style of slavery practiced in various places since the middle of the last millennia.
The type of slavery practiced in North America and parts of Europe during the 16th–19th centuries was an abhorrent, abusive — and sinful — form of slavery rooted in prejudice, hatred, and greed. Forced captivity and physical abuse like that practiced by the slave traders and owners of North America was clearly ungodly and unbiblical since it violated everything Jesus preached concerning love, forgiveness, and charity. Jesus told us to treat our neighbors as we want to be treated, which directly condemns the modern institution of slavery. No argument to the contrary can stand the test of Scripture.
On the other hand, the kind of slavery known in biblical times was usually (but not always) something different. It was typically one of three forms, each of which differs significantly from the forced slavery imposed on Africans and other cultures oppressed by the modern slave trade.
The first and most common form of slavery found in the Bible was a voluntary servitude entered as a result of financial indebtedness (i.e., a bondservant). A person in debt to another person could sell himself into slavery to his creditor as a way of paying off his debt. Sometimes the service lasted for months or years, and sometimes it could last for life. In all cases, the arrangement was voluntary and not forced, and most importantly it was not abusive. While the person enslaved lost all liberty, they were not abused, and in fact, the owner had no incentive to abuse the slave since he had accepted the slave's service in lieu of repayment of a debt. The slave was more valuable if he was healthy, well-fed, and content than if he was abused.
Perhaps the best-known example of this type of slavery in the Bible is found in the letter of Philemon, where Paul encourages a believing slave, Onesimus, to return to his master as required by Roman law. The slave was bound by law to obey his master because he had entered into slavery to pay a debt, so Paul called for Onesimus to return to his master to fulfill his obligation and for the purposes of a good Christian testimony.
Likewise, the Bible commands masters to treat indentured slaves in respectful ways and without abuse:
When a master is cruel and abusive to his slaves, the master is sinning, and all sin is wrong. Likewise, when a slave rebels against the authority of a master whom he agreed to serve, the slave is sinning. So, the primary form of slavery found in the Bible was a type of service similar to someone enlisting in the military today. It was voluntary, often limited in time, not coerced yet irreversible and enforced harshly.
A second type of biblical slavery occurred as a result of men captured during times of warfare. The Bible does not condemn nor oppose the practice of taking prisoners in war, though it does demand that such men be treated with dignity and care.
Finally, a third type of slavery permitted in the Bible occurred when the Lord placed His people into bondage as a judgment, which happened to Israel on several occasions:
The Lord also sent Joseph into slavery for a period of time, as Joseph himself testifies:
As well, God sent all of Israel into bondage during the time of the Babylonian captivity.
So slavery in the Bible was a permissible state of human existence provided that institution was either voluntary, a result of warfare or an act of God's judgment. In all cases, it was very different than the institution practiced in recent centuries in North America in which a person was enslaved merely because of the color of their skin.
Therefore, when the New Testament uses slavery as a picture to describe the relationship between the believer and Jesus, it is referring to the first type of biblical slavery, NOT to the one we typically think about from our history. The Bible is referring to voluntary servitude entered freely because of a debt (i.e., a bondservant or bondslave). Jesus often used a picture of this type of slavery to represent our obligations to serve Jesus well, since He has paid off our debt. Paul also compares believers to slaves in his letters often saying we are "slaves of Christ."
In all three kinds of slavery, the sovereignty of God is always at work placing people in certain walks of life to test and refine them spiritually and giving opportunity for believers to witness to the Gospel under those conditions. Every believer receives a path in life and a set of challenges given by God. Some believers must learn to witness from prisons, others from hospital beds, others from trenches in warfare, others in conditions of poverty, and some will witness from chains as slaves. In the wisdom and providence of God, all these situations can be useful to Him in bringing hearts to faith and Himself to glory.
Nevertheless, the Bible does not require believers (or any person) to remain in an abusive situation. A person subjected to unjust and inhumane circumstances (like that of historic, North American slavery) has the right to seek relief from their circumstances and to work to end such criminal institutions. While suffering for our faith in biblical, there is nothing redeeming about suffering for its own sake, no more than a Christian living in poverty or suffering under an abusive relationship is sanctified merely because they endure such suffering. No one should be required to remain in abusive, unjust circumstances, especially when alternatives are available.