Why doesn't the bible denounce slavery?

Slavery is not denounced in scripture and in fact is allowed. How can we trust a book that is ok with slavery?
You must first understand that slavery does not carry the same definition today that it did in the ancient world. The slavery of the 1700-1800s is not what the bible writers had in mind when the discussed slavery. I’m confident that the bible would call for the abolishion of the African slave trade. Although there were abuses in the ancient world, the type of slavery that was allowed was quite different.

It is interesting to know that every culture without exception until the modern age, has had slavery as part of society. Even though we don’t have an open slave trade, it still exists in various forms. You will probably see from this article that our oh-so-modern society still practices forms of slavery.
All throughout history slavery has been a common part of daily life. 
Why would Jesus not call for the abolition of this abhorrent crime. The answer is that:
The form of slavery that was allowed in Israel was not the abhorrent crime, that the African slave trade was. It was ultimately Christians who realized that the black slave trade was something that God detested, based on the bible.
Before we jump into the specifics, you should know this.
There are many places where the bible is not being prescriptive but only descriptive. Just because David slept with Bathsheba doesn’t mean we should then “go and do likewise.” The bible was simply describing his behavior not prescribing it. Just because the bible says a master beat his slaves, doesn’t mean we ought to. There are cases like Exodus 21.1-11 that are cited as instructions to sell your daughters into sex slavery. That is simply not true. The passage is assuming that the reader understands the structure and meaning of slavery. The section of scripture is designed to preserve the rights of those that would get sold into slavery by family members. WOHHH WAIT!!!! WHAT????
That’s what your thinking right, how could a family member sell another family member into slavery? Wait a second, before you tune out you have to understand the system.
There were different categories of slaves. Most were slaves because of financial debt. There were safeguards against fellow countrymen being enslaved against their will. For instance, If an Israelite kidnapped a fellow Israelite and forced him into slavery that was punishable by death.
Debt slavery was probably the most common form of bondage.
In the ancient world there was no such thing as bankruptcy. However, there was still such a thing as debt. So from time to time -well lets be honest actually pretty often- someone would run out of money. This family, let’s call them the Jones, are in a real danger of starving. To add insult to injury the place where the Jones live is suffering a terrible famine. Father Jones has some difficult decisions to make. He could borrow money to feed his family. As you can imagine, borrowing money to put food on the table is something that only a desperate person would do. Being desperate, the Jones are unlikely to be able to repay this debt when the collector comes around. The Jones go deeper and deeper into debt as the famine stretches on. It’s the kind of crushing, oppressive debt that they could not break out of.
In our modern society, if the Jones get into that kind of debt they would simply file bankruptcy. There are consequences but it’s at least an option. They could be hounded by debt collectors, they could have their credit ruined, they might even wind up in a shelter.  However, no matter what happens the Jones will still be free. In the ancient world there was no such thing as bankruptcy. The depth to which they could fall was much greater. What  options would the Mr. Jones have? He could sell himself into slavery.
This was not slavery that would last forever. It was slavery that would last until his debt was paid off, or until the mandatory slave release date (more on that in a minute). This was not the only way to become a slave but it was the most common. Most people that became slaves in the ancient world were slaves because they had gotten into debt and were basically paying their bills.
What if the debt is too much for Mr. Jones to work off?
In many cases when the debt was particularly bad, Mr. Jones would be forced to sell all of the family members into slavery. Don’t misunderstand this point. Mr. Jones is not abandoning them to their fate. He still had the duty to protect and provide for his wife and children. A good father would only do this in a dire situation, and with discretion. Sometimes, Mr. Jones just had no choice.
You’ve probably heard of arranged marriages. There would also be cases where a daughter would be sold as a bride in order to release the family from debt. A bride price (dowry) was common. Mr. Jones could be pressured into arranging the marriage with the debtor rather than someone else. Mr. Jones could choose to arrange little Sally Jones’ marriage and keep the rest of the family out of slavery, or he could be stubborn and refuse the arrangement and accept the consequences. The passage in Exodus 21 assumes that this type of situation would arise. The passage is in place, not to entice people to sell their daughters into slavery but instead to protect their rights and dignity if it comes to that.
Another thing to understand is that slavery did not mean you had to live in the stables. The slaves of a high ranking or important house or family would have much more political clout and social standing because they were associated with that family. Slaves in the ancient world could be anything from day laborers to doctorate degree holding tutors for wealthy families’ children.
Another interesting thing relating to slavery in the Jewish law was the year of Jubilee. It was established in the Scriptures that every 7 years the slaves would go free. It was a time to celebrate and thank God for freedom. So domestic slaves would only ever serve a single sentence of 7 years. Now whether that is how it happened is hard to say but that was what their law instructed them to do. This doesn’t seem like a system that was designed to dehumanize and mistreat the people it enslaves. It seems like a system that is designed to maintain human dignity and respect while serving the purpose of debt collection.
Often what would happen even outside of the Jewish territories is, a slave would eventually serve out their time, and pay their debt. At that point the slave would be given the choice to go free, or to remain a slave of that household. The prodigal son, in Jesus’ story says that even the slaves in his father’s house have plenty to eat and a safe place to live.  Slavery could actually be a really sweet gig. It could mean plenty of food, clothes to wear, and a warm place to sleep. That is more than many people could expect in the poorer parts of Israel. So a slave could choose to continue to be a slave for that household. If they chose to stay, they would receive an earring by having a nail driven through their ear into the door frame of a house. This symbolized that they were part of that household.
Again it is clear that a system of slavery that anyone would choose to stay in, is not the system of slavery that we have seen in the past few centuries. Of course, there are cases where there were injustices and mistreatment, but that was not the way the system was designed. In fact in scripture, slave owners are guided to treat their slaves fairly and with respect. Paul even guides a well known slave owner to treat his slave as a brother in Christ.
 A better understanding of ancient slavery might be found in the phrase, “bound worker.”
Slaves in the ancient world were of all races. What we have seen in the last few centuries was a slavery based on race.  Slavery based on cruelty and sexual abuse are not condoned by the bible. The slavery of the ancient world and recent modern iterations of slavery could barely be called “related.”
You may say, “what about when the bible tells the nation of Israel to take slaves from the surrounding peoples?” There were slaves that were captured in foreign wars… however, Israel rarely engaged in foreign wars, so that was rare. The thing to remember is that the law told the people to treat aliens with respect and love. The bible is clear that masters should not mistreat their slaves.
I do want to be fair. There were types of racial and sexual slavery in the ancient world. My point is that these types of slavery are not things that God seemed to be ok with.
We have to be careful how we approach the issue of slavery. It has a treacherous history. While it was eventually Christians who fought for the abolition of black slavery, the story has a darker side. At that time, the bible was being used as support for the african slave trade. This is one reason why it is so important to understand what the bible says in it’s context.
Ultimately it was Believers in England who began the process that eventually lead to the abolition of slavery. Christians rammed abolition through Parliament and then it was Britain that sent out it’s gun boats to stop the slave trade. about 11 million Africans were enslaved and transported to the Americas. About 13 million were transported to the middle east. In both cases it was the result of Christians that forced the death of this disgusting endeavor.
Slavery has never been an ideal situation. It’s always been an ugly thing in it’s various iterations. There will be a time when there is absolutely no more slavery. We look forward to the Kingdom of God, where there is no longer any sin. In that new world, there will be no more slaves, but only those who choose to be there.