Google "Paul Titus Slavery" ... I'm surprised that you are unaware of this. Paul said that slaves should not rebel against their masters and that was seen in the bible belt as Paul's endorsement of slavery. Of course now, Christians claim that it was a ludicrous way of looking at it.
The point is that the bible is not as straightforward at it seems. What may seem very straightforward during on era can be seen as a very silly interpretation in a later era.
I am well aware of what Paul said and I also don't think he was endorsing slavery. You are saying that people's interpretation changes. The facts of the Bible haven't changed. Each is free to interpret as they wish. However, that doesn't change what God said or meant.