The bible explicitly endorses two types of slavery....indentured servitude (for Hebrews) and chattel slavery (for non-Hebrews). With indentured servitude, a person voluntarily agreed to sell his labor to his master for a temporary period of time after which the servant would be granted some kind of remuneration. With chattel slavery (the type of slavery that existed in America during the 1800s), the slave was the permanent property of his master. Most Christians acknowledge that indentured servitude existed for Hebrews, so I won't discuss this. I want to concentrate on the slavery that applied to non-Hebrews (i.e. chattel slavery). Below I will show that the Hebrews got their chattel slaves by buying them or capturing them during war.
Leviticus 25:44-46 (NKJV)
44 And as for your male and female slaves whom you may have—from the nations that are around you, from them you may buy male and female slaves. 45 Moreover you may buy the children of the strangers who dwell among you, and their families who are with you, which they beget in your land; and they shall become your property. 46 And you may take them as an inheritance for your children after you, to inherit them as a possession; they shall be your permanent slaves. But regarding your brethren, the children of Israel, you shall not rule over one another with rigor.
Here you can see that Hebrews can buy non-Hebrew slaves as permanent property. This is in contrast to Hebrew indentured servants who entered into a contract with their masters for a set period (7 years). Indentured servants couldn't be bequeathed as inheritance because they were not considered permanent property. Also, notice that this passage makes a distinction between the treatment of Hebrews servants who are not to be treated ruthlessly like non-Hebrews were.
The second way chattel slaves could be obtained is by attacking foreign cities and enslaving the inhabitants:
Deuteronomy 20:10-18 (NKJV)
10 “When you go near a city to fight against it, then proclaim an offer of peace to it. 11 And it shall be that if they accept your offer of peace, and open to you, then all the people who are found in it shall be placed under tribute to you, and serve you. 12 Now if the city will not make peace with you, but war against you, then you shall besiege it. 13 And when the Lord your God delivers it into your hands, you shall strike every male in it with the edge of the sword. 14 But the women, the little ones, the livestock, and all that is in the city, all its spoil, you shall plunder for yourself; and you shall eat the enemies’ plunder which the Lord your God gives you. 15 Thus you shall do to all the cities which are very far from you, which are not of the cities of these nations.
16 “But of the cities of these peoples which the Lord your God gives you as an inheritance, you shall let nothing that breathes remain alive, 17 but you shall utterly destroy them: the Hittite and the Amorite and the Canaanite and the Perizzite and the Hivite and the Jebusite, just as the Lord your God has commanded you, 18 lest they teach you to do according to all their [a]abominations which they have done for their gods, and you sin against the Lord your God.
Here we see that when the Hebrews attacked a non-Hebrew city they made an offer to the inhabitants:
(1) surrender and pay a tribute (i.e. they would be forced to work for the Hebrews) OR
(2) the men would be slaughtered and women/children and livestock taken as plunder.
In case (2), women and children are described as plunder, which is property that is (usually violently) acquired by the victor during a war. Here the Hebrews could march into a house of the conquered city and drag out any women and children and enslave them. These weren't combatants and posed little treat to the Hebrews, but they were of economic value.
Today we recognize that slavery is immoral because slavery, by its very nature, is a violation of a person’s liberty. It reduces people into objects that can be owned. Some apologists claim that slaves were treated with kindness and not abused like black slaves in America were. Even if this was true, this makes no difference to the morality of owning another person as property - slavery was and will always be immoral. Other apologists argue that these laws are no longer in force. Again this is irrelevant. The fact is that there was a point in history where god thought that owning another person as property (chattel slavery) was okay.
My question is, if an omnipotent and benevolent god exists and he gave these laws to humans, why would he condone slavery? A benevolent god and a god that condoned slavery is a contradiction. Either the god of the bible exists, in which case he isn't benevolent or he doesn't exist.
Below is an excellent video which counters many of the objections that apologists have on this subject: