Sawing off the branch we're perched on

I've quoted part of this before, but now I'm going to include a larger excerpt:

Critics of the morality of the God of the Hebrew Bible rarely ask themselves what the source of the morality from whose perspective they present their criticism is. A few years ago, I watched with great pleasure the HOB production called "Rome." The final disk of the DVD version of "Rome" includes interviews with some of the people involved in the production of the program. In one interview, someone or other was asked in what ways he thought the Romans were like us and unlike us. He replied that they were remarkably like us in most ways, but that there was one way in which they were very different from us: in their extreme brutality–in both their willingness to commit brutal acts and in their indifference to the pervasive, entrenched brutality of their world. When he was asked whether he could explain why we and the Romans were so different in this respect, he did not quite answer by saying "Christianity is what made the difference"–I don't think he could have brought himself to say that–but he did identify "Judaeo-Christian morality" as the source of the difference. And it was a very good answer. The morality of almost everyone in Western Europe and the anglophone countries today (if that person is not a criminal or a sociopath) is either the morality that the Hebrew Bible was tending toward or some revised, edited version of that morality. Almost every atheist (in Western Europe and the anglophone countries), however committed he or she may be to atheism, accepts some modified version of what Judaeo-Christian morality teaches about how human beings ought to treat other human beings. And even the modifications are generally achieved by using one part of that morality to attack some other part. (For example, by attempting to turn the principle "don't make other people unhappy" against Judaeo-Christian sexual morality.) 

The morality to which critics of the moral character of the God of the Bible appeal is a gift to the world from Israel and the Church and is by no means self-evident. I don't think that many missionaries have heard anything resembling the following from those whom they were attempting to convert: "Hey–it says here, 'But of the cities of these people, which the Lord thy God doth give thee for an inheritance, thou shalt save alive nothing that breatheth…thou shalt utterly destroy them.' That's awful. How can you expect us to worthship such a God?" And the reason they haven't heard that is that most people in most times and at most places would see nothing but good sense in that command. Most people have taken it for granted that when  tribe or nation moves into new territory it will kill those of the previous inhabitants that it does not enslave. That's what people do–the Old Common Morality says–and they'd be crazy to do otherwise. Peter van Inwagen, "Comments on "The God of Abraham," M. Bergmann et al. eds. Divine Evil? The Moral Character of the God of Abraham (Oxford 2013), 81-82.