The following passages describe how the slaves are to be treated in the Bible.
Old Testament endorsement of slavery
However, you may purchase male or female slaves from among the foreigners who live among you. You may also purchase the children of such resident foreigners, including those who have been born in your land. You may treat them as your property, passing them on to your children as a permanent inheritance. You may treat your slaves like this, but the people of Israel, your relatives, must never be treated this way. (Leviticus 25:44-46 NLT)
If you buy a Hebrew slave, he is to serve for only six years. Set him free in the seventh year, and he will owe you nothing for his freedom. If he was single when he became your slave and then married afterward, only he will go free in the seventh year. But if he was married before he became a slave, then his wife will be freed with him. If his master gave him a wife while he was a slave, and they had sons or daughters, then the man will be free in the seventh year, but his wife and children will still belong to his master. But the slave may plainly declare, ‘I love my master, my wife, and my children. I would rather not go free.’ If he does this, his master must present him before God. Then his master must take him to the door and publicly pierce his ear with an awl. After that, the slave will belong to his master forever. (Exodus 21:2-6 NLT)
When a man sells his daughter as a slave, she will not be freed at the end of six years as the men are. If she does not please the man who bought her, he may allow her to be bought back again. But he is not allowed to sell her to foreigners, since he is the one who broke the contract with her. And if the slave girl’s owner arranges for her to marry his son, he may no longer treat her as a slave girl, but he must treat her as his daughter. If he himself marries her and then takes another wife, he may not reduce her food or clothing or fail to sleep with her as his wife. If he fails in any of these three ways, she may leave as a free woman without making any payment. (Exodus 21:7-11 NLT)
When a man strikes his male or female slave with a rod so hard that the slave dies under his hand, he shall be punished. If, however, the slave survives for a day or two, he is not to be punished, since the slave is his own property. (Exodus 21:20-21 NAB)
New Testament endorsement of slavery
Slaves, obey your earthly masters with deep respect and fear. Serve them sincerely as you would serve Christ.(Ephesians 6:5 NLT)
Christians who are slaves should give their masters full respect so that the name of God and his teaching will not be shamed. If your master is a Christian, that is no excuse for being disrespectful. You should work all the harder because you are helping another believer by your efforts. Teach these truths, Timothy, and encourage everyone to obey them. (1 Timothy 6:1-2 NLT)
The servant will be severely punished, for though he knew his duty, he refused to do it. “But people who are not aware that they are doing wrong will be punished only lightly. Much is required from those to whom much is given, and much more is required from those to whom much more is given.” (Luke 12:47-48 NLT)