As I pointed out in previous posts, slavery was defended by many conservative Christians in the 18th and 19th centuries. These Christians were not only southerners. Charles Hodge, for example, from Princeton Seminary defended the practice. It seems that most Calvinists did defend it. Three vocal Calvinist defenders were George Whitefield, R. L. Dabney and James Petigru Boyce. Those who spoke out against slavery tended to be non-Calvinists, for example John Wesley, Charles Finney, George Fox, and so-on.
Abraham Lincoln in his second inaugural address said: Both [the pro-slavery side and the abolitionists] read the same Bible and pray to the same God, and each invokes His aid against the other.
My point is that the Bible was not really clear on this important moral issue, just as it is not clear on much of what it teaches, including how one is to be saved. It seems that if the Bible were actually a divine revelation, an omniscient being could have done a better job of making himself clear.
Below is a 15 minute section from the documentary, God in America, which details how American Christians used the same Bible to both defend and condemn slavery.