People's concept of morality evolves as human society evolves. Conservatives -especially the religious - are stuck in the morality of the ancients. Progressives are continually evolving morally. That's not to say that progressive morality is always right, but they are more right than wrong.
For example, given the morality of the ancients, not even Jesus spoke against slavery. Yet as humanity progressed it's become almost universally accepted that slavery is immoral. Modern government - formed by progressives (for their time) were WAY ahead of religious institutions in condemning slavery.
Of course many conservatives have a hard time conforming to the morality of the ancients much less modern morality.
Apparently you have not read the Bible. There was a man named Moses, ever hear of him? If not, God raised him up to free the Hebrew nation from slavery.
But he was not done. Although he allowed for slavery in the Hebrew nation, it was used more as a way to survive for the down and out who were not linked to a tribe to help them survive. On top of that, they were to be released from slavery after 7 years of service.
But it does not end there. Moses even implemented a day of rest, even for slaves, something unheard of in the ancient world that knew work and toil 24/7.
So thank Moses for "evolving" away from slavery. No, thank the God that pulled the strings behind the curtains.
As for Jesus, he said that those who sinned were a slave to sin and that he came to set the captives free. What he was saying was equally revolutionary, if not more so. To think that God could set someone free from their internal chains despite maybe being a slave is astonishing and accurate.
I fully believe that Christian slaves back in the day were much happier than inwardly tormented souls like emperor Nero even though they had all the money and "power" anyone could want.