Slavery has been in existence on every continent throughout history. The Greeks and Romans owned slaves. Egypt's dynastic achievements were built on the backs of slave labor. Defeated peoples served their new rulers as slaves. In the 1500's slave trade was institutionalized with most slaves coming primarily from Africa. Powerful tribes sold weaker tribal members into slavery. Eleven million Africans were exported worldwide into slavery.
Jews became slaves and they too owned slaves:
Leviticus 25:44 (ESV) "As for your male and female slaves whom you may have: you may buy male and female slaves from among the nations that are around you. You may also buy from among the strangers who sojourn with you and their clans that are with you, who have been born in your land, and they may be your property. You may bequeath them to your sons after you to inherit as a possession forever. You may make slaves of them, but over your brothers the people of Israel you shall not rule, one over another ruthlessly."
Yes, God allowed, but didn't ordain, slavery. Slavery was due to disobedience of mankind. It's a shame that we ever had slaves, but then to, it's a bigger shame that man ever sinned. Slavery was even an early means of currency:
Exodus 21:20-21 (ESV) “When a man strikes his slave, male or female, with a rod and the slave dies under his hand, he shall be avenged. But if the slave survives a day or two, he is not to be avenged, for the slave is his money."
Since slaves had value they were to be treated with great care. Just as a master is told to treat his beasts humanely, the same applied to slaves. Slave masters were forbidden to be cruel:
Colossians 4:1 (ESV) "Masters, treat your slaves justly and fairly, knowing that you also have a Master in heaven."
Furthermore slaves were to be good servants:
Titus 2:9 (ESV) "Slaves are to be submissive to their own masters in everything; they are to be well-pleasing, not argumentative, not pilfering, but showing all good faith, so that in everything they may adorn the doctrine of God our Savior."
Colossians 3:22 (ESV) "Slaves, obey in everything those who are your earthly masters, not by way of eye-service, as people-pleasers, but with sincerity of heart, fearing the Lord."God allowed slavery for a time just as he allowed divorce and multiple wives. It's not that God ordains divorce, adultery and slavery; but that our hearts are calloused. See here what he says about those who enslave others:
1 Timothy 1:10 (The law is not made for a righteous man, but for the lawless and disobedient): "The sexually immoral, men who practice homosexuality, enslavers, liars, perjurers, and whatever else is contrary to sound doctrine..."
You can readily see that those who would make a slave of another are equivalent to homosexuals, liars, and the sexually immoral.
Why does God allow bad things to happen to mankind? It's because man chose to follow the ruler of the world (not meaning that the slaves were the disobedient, but that the institution is because of man's disobedience):
Ephesians 2:2 "Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience..."Satan is the prince of the world. John 12:31 supports the ruler of the world to be Satan. Just as God removed his protective hedge from around Job, since mankind was disobedient, he removed his protective hedge from around everybody. Just as Job was faithful and still suffered persecution, those in slavery may have been faithful, they too were persecuted. Slavery is not of God, but he allows mankind to suffer our own devices! Just as Jesus said that the poor will always be with us, servitude will be as well! It may not be ownership, indenture or pecuniary slavery, but slavery will always exist.
Jesus didn't come to free mankind from earthly masters, but to free them from sin and fear of eternal death! With one word Jesus could have ended poverty and obliterated slavery. However, he chose to leave them alone! Why?
It's no shame to be a servant. In fact we're all to serve!
1 Corinthians 6:19-20 (ESV) "Or do you not know that your body is a temple of the Holy Spirit within you, whom you have from God? You are not your own, for you were bought with a price. So glorify God in your body."
Christians are all slaves! We're slaves to God. Spiritual slavery is a duty and an honor! Likewise, human slavery is a picture of our relationship to Christ. In our eyes it's a great injustice, but for the Lord, the servants to man are a special people. His own chosen people were servants to Egypt. Egypt was a picture of slavery to sin and the Pharaoh was a picture of Satan. The Hebrew people were delivered from slavery (sin) in Egypt and became a mighty nation in their own right! God allowed his people to be slaves, and he had a reason:
Matthew 5:5 "Blessed are the meek: for they shall inherit the earth."
Servants are an example of meekness. Good servants are a better example. Those who suffer slavery will get their reward. There will be a special crown for those who serve others, but slavery has another reason as well. Persecution spreads the gospel message!
If Paul had never been imprisoned Christianity would never have spread so rapidly. If Peter hadn't been crucified upside down, Christianity would not have grown. Injustice breeds growth. Just as more Christians suffered under Nero Caesar and the gospel message spread, likewise slavery spread the gospel to a distant dark continent and it's indigenous peoples. God used the persecution of human beings as a means to lead them to Christ! His desire was not slavery. That was Satan's desire, just as persecution of early Christians is of Satan. However, God used this crooked stick to allow all peoples to hear his word.
Today, those descendants of slaves are a glorified righteous people on the whole. They worship the same Christ that we worship and salvation is for all! Without persecution, many peoples of the world would never know of the grace of Jesus!
Biblical support for slavery can never be justified any more than biblical support for divorce nor adultery. God's will is that all men shall be free. Man was created a free creature, not even a servant of God, but a willing worker in his garden; a friend! Adam's disobedience put the curse on the earth. Man was cursed with toil in tilling the earth. Before the fall of man the earth was fruitful and God provided all the needs of mankind with little effort on the part of man.
With hardship in tilling the soil came slavery. It's one of the oldest institutions known. Man, because of the deception of Satan, is responsible for slavery. It's not God's will that man serve others, but to serve him alone!
Heaven is a wonderful place. Those who suffered slavery here on earth will sit at the table of angels and glorify God who finally makes them free. "Free at last. Thank God, Almighty. Free at last!" (Martin King).