One of the greatest mistakes religion ever made was preserving their traditions in book form. Contrast the bible with something like an economics or physics textbook. If the collective knowledge of the field updates, they can edit an old textbook or rewrite a new one from scratch and the old one falls away, becoming only useful to historians of the field. But if you put special importance on a randomly selected physics textbook from the 1930s and demanded that everyone follow that as a starting point, progress in the field would slow dramatically.
If there were an oral tradition in Christianity that normalized slavery (because at the time slavery was normal) but then the collective morality of society improved as people realized that the world was better off without slavery, they could simply stop reciting that part of the tradition, or even re-write it to say the opposite of what it used to and they wouldn’t have the problem of venerating a book full of immoral teaching.
This is a great animated sketch from Prophet of Zod demonstrating what happens when you have a holy book you can’t edit that you’re forced to defend as a source of moral wisdom: