Slavery is a perfect example. There is a reluctance to accept the position that churches and Christians held on this matter until recently in human history or to acknowledge that, “you know what, if you read what the NT says about slavery, slave owners had a point.” But do Christians sit and argue over slavery? Sam Harris is right; slavery is the easiest moral question we have ever faced.
I wouldn't agree that it's an easy moral choice at all. The "right of the conquered" has a lot of logical moral purchase before you leave the context in which it ceases to be seen that way. Slavery was also the fundamental underpinning of society in Antiquity, on all levels: economic, political, and, yes, moral, as shown by the Greek philosophers. The idea that slavery is fundamentally morally wrong is actually an exceptionally new idea (less than 200 years old, for the most part), and we only all share it because we grew up in a social context that believed that idea to be true. The fact that we have no (0, none, nada, etc.) arguments for the absolute abolition of slavery surviving from Antiquity speaks volumes. People may have thought that freeing slaves was a pious act, but no one wanted to get rid of the institution.
Really, it was only the rise of the modern industrial economy that killed slavery more than any moral arguments against it. And it was the advent of feudalism and the increased sense of semi-free serfs over slaves that brought down Antiquity's slavery. Moral considerations were window dressing.
There were, at the very least, strong arguments against slavery by the ancient monotheists, for example by
Gregory of Nyssa, or in Philo's description of the Essenes:
Ulpian, echoed in the Justinian Code, viewed slavery as unnatural, even if tolerated. The Natural Law tradition of all persons as equal by birth, and its corrollary in spiritual brotherhood and equality, took a long time to develop into an unambiguous condemnation of slavery, but the seed was there. The changing economic and political conditions in medieval Europe allowed the anti-slavery view to be implemented, but it didn't invent it. Certainly moral considerations were paramount in the end of slavery after its revival, and it had opponents calling for its abolition almost as soon as it began.
