I think you're mistaken about the what the Bible says, because both the Old and New Testaments make clear that slaves are to obey their masters, and no writing included in the Bible called for the freeing of slaves in contemporary society where chattel slavery was omnipresent. But even if the Bible DID clearly condemn slavery, that wasn't the point of my original comment.

The Bible contains all sorts of texts that Christians don't follow. The Bible is nothing like a manual for how to be a Christian. That's why there are 10s of thousands of Protestant Christian denominations alone, not even counting Apostolic Christianity like Roman Catholicism and various eastern Catholic and Catholic rite Churches.

You can pretty much support anything you want from the Bible by picking and choosing.

My point was that Christians in massive numbers used their Christian belief systems to uphold slavery in the U.S. South, and they later did the same to uphold Jim Crow, school segregation, the criminalization of interracial marriage, etc. I'm talking about within living memory. I personally sat in pews as a child and teenager in quite mainstream Baptist churches in Ohio, Indiana, Alabama, and Iowa -- listening to several different pastors preach that God intended for the races to be kept apart.

This was not at the time unusual or shocking. Very many (perhaps even most) white Christians simply took that traditional racist teaching for granted.

It's true that prior to the U.S. Civil War, a loud minority of Christians in the North vigorously agitated for an end to slavery, but unlike some commenters in this thread have claimed, that was far from the majority Christian position, and the abolitionists (as they were called) were as inspired by secular philosophy (John Locke, Rousseau, and more) as by Christian theology. In fact, abolitionist Christians CHANGED traditional Christian theology by combining it with secular philosophy in order to get the results they wanted, i.e. an end to chattel slavery.

It's notable, however, that their principled stand usually did not extend to supporting full human and civil rights for Black people, be they former slaves or otherwise.

I illustrated that by discussing the racist sermons I heard with my own ears in very mainstream churches in the 1970s and 1980s.