In our politically correct culture, slavery is easy target practice for unbelievers, who readily point out that the Bible talks about slavery. This is especially true of the Old Testament, which gives pre-Christian Israel various laws on how to obtain and treat slaves. Of course, the entire issue is exaggerated when they do so. They conveniently ignore the fact that slavery was a voluntary process, whereas kidnapping was punishable by death (Exodus 21:16; Deuteronomy 24:7). They overlook the fact that slaves which escaped were not to be returned to their masters (Deuteronomy 23:15). They even like to misquote various verses about being "allowed" to beat slaves within an inch of their lives, when in actuality it's a question of whether the slave is to be fully released with provisions at the master's expense, or whether the master is to be tried for murder. But even when we look at all verses in context, it's clear that slavery does exist in the Bible. Thus, Christians still have something to answer for.
Or do we? The first thing to understand about any moral issue is that God is the standard. There is literally no higher authority than Him, and so He could, if indeed He desired to do so, come out tomorrow and justify the most repugnant of slave practices. Those who disagreed would then become wrong. But what we find in reality is that the exact opposite is the case.
See, one thing unbelievers fail to recognise is that just because God allows something doesn't mean He's ok with it. When you read the Old Testament, you are reading the Old Testament. You're not looking at the finished cake, you're looking at the eggs being cracked and put in the bowl. Begin in Genesis 1, the earth is "without form and void". By the end of the chapter, there is land and sea, moon and stars, life in every corner, a man and his wife, "And God saw every thing that he had made, and, behold, it was very good."
Just as Genesis 1:2 is not the end of the creation account, so also are Old Testament slave laws not the end of God's reformation of mankind. Unfortunately, slavery has been a common practice throughout human history, and it even continues to this day. The laws we see in the Old Testament seem extremely unfair to us (mainly because Christians, motivated by the Bible, helped put an end to slavery in the Western world), but to the ancient world, they were extreme against slavers.
But we no longer have just the Old Testament. History progressed, God Himself entered creation as a man, and He told us all sorts of things, including "...Moses because of the hardness of your hearts suffered you to put away your wives: but from the beginning it was not so." (Matthew 19:8). See, even in the Old Testament, God made it quite clear that He hates divorce (Malachi 2:16). Nevertheless, Jesus says, He regulated it, because there was no feasible way to stop it from happening.
In much the same way, God is not the common denominator in slavery, man is. God did not find a culture that had no slaves and say "hey, you guys have slaves now". Rather, He found a culture made up of slaves, in a world filled with slavery, and basically said "right, I know you're going to practice slavery, but you're going to do it on my terms. Remember what it's like to be slaves. Don't you dare be like your former masters." And He cleaned the practice up. This was not the end.
In the New Testament, we notice a change. Since God was no longer addressing Israel as a nation, but rather the whole world, He continues to address the issue of slavery, but not in a "this is how it should be done for now" kind of way. Rather, first of all, He continues with His belief that it should be voluntary. Kidnappers, if they die in their sins, are guaranteed an eternity in Hell (1 Timothy 1:10). Morally, this eliminates all other slave ethics. You don't need to worry about how to treat slaves if you literally cannot obtain one.
Nevertheless, God knows He is not addressing a believing world. Slavery would exist for another 2,000 years, and in all likelihood will continue until Christ Himself reigns. Therefore, it speaks to both masters and slaves alike. To masters, the charge is this: If you're harsh to your slaves, God will be harsh to you (Ephesians 6:9). But here's the interesting part: Slaves are to consider themselves equal with other men, but if they can gain their freedom, they are encouraged to do so (1 Corinthians 7:21).
The New Testament does give slaves instructions on how to live as slaves, including being obedient and kind to their masters. This is not an endorsement of the practice. Rather, it is like telling you to give your wallet to a man who points a gun at you. Better to lose your wallet than your life. In the same way, the New Testament has a greater goal.
Of course, there is a place for Christianity in politics. According to Romans 13, the entire purpose of government is to be a minister for good and a terror to evil. Therefore, if the Church has any say in the matter, we should seek to abolish practices such as slavery. And again, we did. But the primary goal of scripture is the spread of the Gospel. Therefore, whether Christians live free in the West, or in bondage in places like China, we aren't supposed to spend as much time and effort fighting for this world as we are preparing ourselves, and you, for the next.
At the end of the day, you can free a slave, but if you don't give him the Gospel, he's captive to sin, and will spend eternity in Hell. But give the Gospel to a slave, he may spend his life in bondage, but he will forever be free in God's Kingdom. The question you need to ask is which are you? Are you a slave to sin, or are you the Lord's freedman?