The Bible & Slavery (A Synopsis)

The apostle Paul condemned slave-trading as sinful (1st Timothy 1:9-10, "kidnappers" in the NKVV). Slave-trading is a sin that would have gotten a Christian ejected from the local assembly.  Christianity's basic principles (e.g. Galatians 3:28) undermine involuntary slavery.

He commanded slave Christians who had the opportunity to be made free, to use the opportunity (1st Corinthians 7:21b).  He also forbade Christians from resorting to indentured servitude/voluntary slavery for any reason (1st Corinthians 7:23).

Because Christianity forbids the use of violence to overthrow the government (Romans 13:1-2), the New Testament treated slavery as a fact of life. However, Christians have a collective history of buying slaves out of servitude, and legally advocating against slavery. 

The New Testament is not a set of social-policy writings, and Christianity is not a political philosophy. So, for critics to demand that the New Testament address specific political or social policy issues of their own concern is wrong-headed. The New Testament also wasn't written to address early 20th-century arguments over the merits of the gold standard, or if women should get the vote.

God in the Old Testament approved involuntary servitude for a number of reasons, in a social context unlike their pagan neighbors, and that of the New Testament:

1.  The OT taught that everyone is made in the image of God, including slaves. This is a radical difference from pagan slavery, which defined slaves as non-persons.

2.  Israel's slaves got the Sabbaths and festivals off along with everyone else. Exodus 20:10, Dt. 16:10-11.

3.  A master who injured his servant had to let the servant go free. Exodus 21:26-27. 

4.  Murdering a slave got you the death penalty, Ex. 21:20.

5.  Slave-trading got you the death-penalty. Ex. 21:16.

6.  The OT rules about slavery were not based on racial inferiority or superiority (e.g. the Nazi principle that Aryans are better than all other races). The OT Jew was commanded to treat wayfaring strangers and immigrant workers like neighbors. Non-Jewish visitors and residents were allowed to participate in Passover. 

7.  OT slavery was often because of debts, or the person was a prisoner-of-war (though these were not the only circumstances you find in the OT for enslavement). When a person's debt was paid off, they were supposed to go free. Moses commanded the death-penalty for anyone who hunted, captured, and/or sold people for slavery

8.   Moses' jubilee rules demanded that all debts be cancelled and all Hebrew slaves be set free every seventh year (Exodus 21:12). This rule did not apply to non-Hebrew slaves, however. God in the Old Testament system used a favored-nation ethic (God accorded certain privileges to the Jews He did not accord to others) that He later abolished in the New Testament.

9.  Slaves in Israel were allowed to run away, and could not be returned to their master!  Deuteronomy 23:15. This was a legal incentive for masters to treat their slaves fairly and decently.

In short, slavery in Old Testament slavery was not chattel slavery (chattel slavery is the system in which the slave is nothing but property, has no rights, and the master can treat the slave any way he pleases). Israel had slave laws because Israel was a geo-political nation involved in warfare and international commerce, which the Christian Church is not.  The New Testament deals with the Roman slave system, treating it as a social given that had to be dealt with relationally and religiously, not militarily.