The Roman world in the first century AD was completely different from what we find in the Mosaic Law and ancient Judaism. I don’t mean to imply that ANE nations surrounding Israel had no slavery. They did, but the New Testament reaches out to foreign nations—i.e. gentile nations, and is not only concerned with the Jews. Therefore, the social structures of the gentiles are laid bare and God through the preaching / writing of the New Testament begins to confront them, exposing the wrong and pointing to right behavior. Slavery in 1st century Rome is an institution, in fact, it is claimed that 85 to 95% of Rome’s population were slaves! Some Biblical critics seem to believe that, because Jesus didn’t equip his disciples with an opposing economic plan that he never said anything explicit against slavery, but they are wrong. From the very first day of his public ministry Jesus pointed out what he had set out to do; namely, “… to proclaim release for captives and …to set free the oppressed, (Luke 4:18 Moffatt; cf. Isaiah 61:1).
Just as in the Old Testament, God worked with an imperfect reality, but pointed to the ideal. The reason for this is that God works on our hearts. He values our freedom—i.e. freedom of choice or freewill. He wants us to change our ways by returning to him and receiving him as our Lord and King. He is not so much interested in things like slavery, war and murder as he is in the attitudes that have created such terrible evil—greed, envy, divisiveness etc. These are heart issues, and if man’s heart can be changed, then his world would be changed. For example, Christian masters were to call their slaves brother or sister (Philemon 1:16). One can hardly read Paul’s letter to Philemon without understanding that the whole tone of the master / servant institution stands broken in Christ. For, in him there is neither Jew nor gentile, neither slave nor master, neither male nor female—all are one (equal) in Christ (Galatians 3:28; cf. Colossians 3:11).
Rather than seeking to oppose the Roman laws, Paul addressed the hearts of men on the issue of masters and slaves. Jesus claimed that he was not King over this present worldview (John 18:36). In other words he wasn’t in competition with Caesar. Jesus was not seeking to make war with him or defeat him. Rather, Jesus’ Kingdom (John 18:37) concerned the heart where truth is considered and valued. In Ephesians 6:5-8 Paul told believing slaves to obey their human masters as though they were serving Christ. As for the masters they were to treat their slaves in the same manner, knowing that Christ was the Master of both bond and free (Ephesians 6:9), which being so, we have Jesus saying “Do unto others as you would have them do unto you” (Matthew 7:12). It would be difficult for the institution of slavery to continue in a world that embraced such an attitude. Nevertheless, it doesn’t exist at all in the Kingdom of God.
Both Peter and Paul are criticized by some, because they never clearly condemned slavery as an institution or commanded Christian masters to release their slaves. For one thing, had they done that, the Romans would have certainly outlawed Christianity. Remember, Christianity was not the politically powerful institution that it was in the Middle Ages. It was a budding group of people united in their belief in Christ. Nevertheless, concerning Paul, he renounced slave trading (1Timothy 1:10 – under the guise of kidnapping); he confirmed the equality of slaves, showing no one is inferior to another in Christ (Galatians 3:28; Colossians 3:11); he encouraged slaves to gain their freedom whenever possible (1Corinthians 7:20-22).
We are not called out of this world in order to make war on this world. This world and all the things dependent upon it will pass away without our efforts to destroy it. We need to concern ourselves with the things of God, and God is concerned with man’s heart—with our rebellious attitude toward him. If our hearts are changed, our behavior will change. If our behavior is changed, then our world will change. To seek to outright destroy the institution of slavery in 1st century AD, where 85 to 95 % of Rome’s population included slaves, would be seen as an effort to overthrow Caesar. Consider how difficult it had been for the South in 19th century America to accept the emancipation of their slaves. Could slavery in America ever have been destroyed by a politically weak institution? How less could this be done in the 1st century AD when our numbers were few?
 As I said HERE, this current theme about “making sense of the Old Testament God” is based upon the book: Is God a Moral Monster by Paul Copan. These are my thoughts about his book. He may or may not agree with the impression his book has made upon me, but I thoroughly enjoyed reading what Paul wrote and recommend his book to anyone who is looking for a good read concerning defending our faith.