Almost all modern Christians passionately oppose slavery. Most, no doubt, oppose slavery more than they oppose gay marriage... especially the "horrible" race based slavery of the antebellum American South.
Most of the people I knew at my conservative congregation in Ohio opposed slavery by arguing that God intended for it to slowly be eradicated as part of his progressive plan for the development of fairer human interactions. These conservatives never recognize that their argument is used by liberal Christians to argue for the same kind of slow progressive development in God's plan for things they violently oppose like homosexual marriage.
Because this argument is so popular among orthodox (conservative) Christians it's worth exploring in more detail.
Among the problems with this reasoning is that it's never deployed outside the slavery question, and this is complicated by the lack of a good definition of slavery. Slavery opposing Christians usually babble vaguely about the "obvious" evil of racism, and the unjust nature of keeping people in bondage for life.
Normally, anti-slavery Christians provide no direct scriptural references to support their beliefs; which is probably because none of the Bible passages that deal with the institution of slavery condemn or even discourage it. On the contrary, the Bible often speaks favorably of it, and even mandate it.
It's my belief that contemporary American Christianity has no coherent theological thinking about the issue of slavery. The sole exception is perhaps the most extreme forms of egalitarian Marxist Christianity which at least maintains a consistent progressive agenda regarding all of Christian theology.
Almost all modern Christian thinking about the issues of race and slavery is the result of hysterical and irrational reactions to the "sin of racism." What is racism? It's whatever left wingers decide it is. In practice, Christian slavery theology (what little there is) is a poorly thought out attempt to avoid accusations of racism. The problem is that what constitutes grounds for racism accusations is constantly changing.
The primary problem with the idea that slavery evolved into a sin over the course of history is that this opens the possibility that other sins have evolved into existence after the Bible was written. If this is possible, Christianity is a fluid and unreliable moral guide.
If slavery is a sin it's the only one never mentioned in the Bible, and if there are sins which aren't mentioned in the Bible than we might be doomed on Judgement Day because God hid important information from us. No orthodox Christian thinker until the modern era has regarded slavery as wrong. Are they all guilty of doctrinal error?
The second problem with the argument that God progressively phased out slavery is that it logically follows that God intends to phase out all unequal power structures. In other words, he's attempting to eliminate all hierarchy and authority.
Is there really a difference between the slave/master relationship and the employee/boss relationship? Perhaps there is, but it's only a matter of degree rather than substance. In politics, Christians encounter the same problem. The feudal system of the Middle Ages needs to be condemned because it mirrored slavery in all important aspects. Should monarchies be abolished because the king has massive authority over the lives of subjects? Are men not to have authority over their wives or children?
The institution of slavery is merely one descriptor of a particular relationship, among the hundreds in human life, that operates on the principles of power, authority, and hierarchy. To declare slavery immoral is to declare that there is something inherently immoral about all these other relationships as well.
Among the ways to avoid the above logical conclusion is to claim race based slavery, like in the American South, is somehow uniquely wrong. However, there doesn't seem to be any way of Biblically arguing this point because God specifically established race based slavery in Leviticus and made numerous socio-political decision based upon ethnicity and race.
Ultimately, the question must be asked: why is basing a decision on race wrong? Is there any reason to believe a Christian has the moral obligation to ignore race when making social decisions?
The modern obsession with being race neutral is morally unjustifiable. There is no reason to ignore race or ethnicity. God didn't create different races so that he could then force humanity to ignore them.
Race is nothing more than extended family. People of the same race are always more closely related to someone of their own race than to someone of another race. God did not create family units and extended families and expect us to ignore them. The most obvious proof for this is God's special relationship with the extended family of Jacob.There is no justifiable reason to view race based slavery as practiced in the American South as sinful.
Of course, with all human relationships abuse leads to sin. Abusing one's slave has always been a sin just as abusing one's wife has been, but there is no reason to believe that the institution of slavery has evolved into a sin during modern times.