"Slavery" is a prime example of "injustice and oppression": In the colonial days of the New World and for some time after that, people were kidnapped and forced into a life-long imprisonment, viewed as less than human, subject to harsh treatment, and had no realistic hope of escape or rescue. Even the "justice" system was against them, and if they managed to escape to freedom, they would find the law against them.
But in the Bible, someone who kidnapped a person to enslave him was the one on the wrong side of the law.
Moses' Prohibition of the Slave Trade
It is plain that the morality taught by Christ -- to do unto others as we would have them do unto us -- would necessarily abolish all forms of oppression, if his words were taken to heart. But what is less well-known is that the slave trade as we know it was actually prohibited under the law of Moses, and carried no less than the death penalty.
Anyone who kidnaps another and either sells him or still has him when he is caught must be put to death. (Exodus 21:16)The type of slavery we know from the colonial era of Europe and the immediate post-colonial era in the U.S.A., which is the slavery of people kidnapped and sold, would have seen the kidnappers and profiteers condemned to death by the Old Testament law.
Even though the Law of Moses called for the death penalty for human traffickers, there was still a different kind of thing also called slavery: commonly when someone sold himself temporarily to pay off his own debts. It was an ancient system for handling bankruptcy. Under Moses' law, this type of slave was released every the seventh year when debts were forgiven. It could be either for a person who had mismanaged a debt or for a thief. When a thief could not repay what he had stolen, with restitution, he would also be forced into temporary slavery to work off his debt. After the seventh year the slave was returned to freedom if he desired it, or allowed to remain with his new household if he found his situation was better off in his new household. Under these circumstances, temporary slavery was a matter of economic justice; it could not become permanent without the debtor's consent.
It is easy to criticize the Old Testament system of working off a debt because it was called by the word "slavery", even though that did not mean either "kidnapping" or "lifelong imprisonment" or even "injustice" in the case of working off a debt. But it is questionable whether our current system for handling bankruptcy and debt is better than working off the debt for a reasonable length of time then having the balance forgiven. Our modern system has the potential for endless debts that are never forgiven and follow an estate beyond the grave, creditors who are never repaid for their bad debts, and people who have been robbed but are never repaid for their losses. We also don't have an effective way to handle that rare person who never does figure out how to manage his own finances or coordinate his own employment. The older system sought out the worker's consent, but with his consent it could make the arrangement permanent, to work for this particular person in return for food and shelter.
Slavery of Foreigners
Moses' slavery code seems most at odds with Christ's teaching of love when it comes to the different status of slaves bought from other nations or taken as prisoners during or after a war. In this light, I think a Christian should take direction from the fact that the Christ's job description included proclaiming liberty to the captives (Isaiah 61:1). That makes its own comment, in a way, about whether the practice of keeping captives was good in God's eyes.
The New Testament Condemnation of the Slave Trade
Many people are aware that, in the U.S.A.'s own arguments over the abolition of slavery, the Bible was often quoted on both sides. It's easy to see that the authors of the Bible took slavery for granted. It's less easy to remember that they lived in a world where "slavery" didn't necessarily mean "kidnapping" or "life-long", where the good guys thought the slave trade was a capital offense, and where many slaves were in their situation temporarily to work off a debt.
The slave trade was condemned explicitly, not only in the Old Testament, but also in the New Testament, as Paul writes Timothy about the lowest of sinners:
... for adulterers and perverts, for slave traders and liars and perjurers -- and whatever else is contrary to the sound doctrine ... (1 Timothy 1:10)
What finally finished slavery in Christian lands was not the explicit prohibitions against the slave trade in the Bible. That, in itself, is testimony to whether the people were reading the Bible to learn from it or using it to justify themselves. What finished slavery in Christian lands was ultimately Jesus' teaching that we should do unto others as we would have them do unto us. The longer people had that saying in their conscience, the less we could tolerate oppression. That is to say, it took a change of heart and a change of conscience before we could recognize that our system of slavery was wrong.