The arguments speak of Christians and many of the scriptures quoted were prior to Christianity, i.e. "The Old Testament." It might be of interest to note that while slavery was a fact of life at the time the scriptures were written (by God or whoever), what was asked of slaves and masters in the New Testament was rather cutting edge. The author of this page leaves out the verses admonishing masters to treat slaves fairly and to acknowledge that slaves are legitimate children of God. In early American history, masters conveniently left those passages out in church services, but they would have provided some thread of hope for slaves seeking freedom as they were preached in slave only churches. Slaves could identify with the plight of the story of Israelites in Egypt. A fuller reading of The New Testament and the context in which it was written certainly led to some enlightened thinking on the part of the free classes of people who began to see that slavery was inconsistent with Christian living. This is why civil rights movements that have borrowed passages of scripture for their causes have been successful in drawing support.
A word of caution to Atheists would try prooftexting contests with Christians vis-a-vis morality coming from the Bible: Know the context of the writing, know writings written around the same time. Don't take the texts of the Bible literally as some Christians do. I have seen that happen with disasterous results.