I do not deny that humanity's powerful tendencies towards in-group loyalties and out-group hostilities would exist even in the absence of religion. Fans of rival football teams are an example of the phenomenon writ small. Even football supporters sometimes divide along religious lines, as in the case of Glasgow Rangers and Glasgow Celtic. Languages (as in Belgium), races and tribes (especially in Africa) can be important divisive tokens. But religion amplifies and exacerbates the damage in at least three ways:

The village of Glenarm in Northern Ireland is the seat of the Earls of Antrim. On one occasion within living memory, the then Earl did the unthinkable: he married a Catholic. Immediately, in houses throughout Glenarm, the blinds were drawn in mourning. A horror of 'marrying out' is also widespread among religious Jews. Several of the Israeli children quoted above mentioned the dire perils of 'assimilation' at the forefront of their defence of Joshua's Battle of Jericho. When people of different religions do marry, it is described with foreboding on both sides as a 'mixed marriage' and there are often prolonged battles over how the children are to be brought up. When I was a child and still carried a guttering torch for the Anglican Church, I remember being dumbfounded to be told of a rule that when a Roman Catholic married an Anglican, the children were always brought up Catholic. I could readily understand why a priest of either denomination would try to insist on this condition. What I couldn't understand (still can't) was the asymmetry. Why didn't the Anglican priests retaliate with the equivalent rule in reverse? Just less ruthless, I suppose. My old chaplain and Betjeman's 'Our Padre' were simply too nice.

Sociologists have done statistical surveys of religious homogamy (marrying somebody of the same religion) and heterogamy (marrying somebody of a different religion). Norval D. Glenn, of the University of Texas at Austin, gathered a number of such studies up to 1978 and analysed them together.¹⁰² He concluded that there is a significant tendency towards religious homogamy in Christians (Protestants marry Protestants, and Catholics Catholics, and this goes beyond the ordinary 'boy next door effect'), but that it is especially marked among Jews. Out of a total sample of 6,021 married respondents to the questionnaire, 140 called themselves Jews and, of these, 85.7 per cent married Jews. This is hugely greater than the randomly expected percentage of homogamous marriages. And of course it will not come as news to anybody. Observant Jews are strongly discouraged from 'marrying out', and the taboo shows itself in Jewish jokes about mothers warning their boys about blonde shiksas lying in wait to entrap them. Here are typical statements by three American rabbis:

Rabbis who will agree to officiate together with a Christian priest are rare, and much in demand.

Even if religion did no other harm in itself, its wanton and carefully nurtured divisiveness — its deliberate and cultivated pandering to humanity's natural tendency to favour in-groups and shun out-groups — would be enough to make it a significant force for evil in the world.

This chapter began by showing that we do not — even the religious among us — ground our morality in holy books, no matter what we may fondly imagine. How, then, do we decide what is right and what is wrong? No matter how we answer that question, there is a consensus about what we do as a matter of fact consider right and wrong: a consensus that prevails surprisingly widely. The consensus has no obvious connection with religion. It extends, however, to most religious people, whether or not they think their morals come from scripture. With notable exceptions such as the Afghan Taliban and the American Christian equivalent, most people pay lip service to the same broad liberal consensus of ethical principles. The majority of us don't cause needless suffering; we believe in free speech and protect it even if we disagree with what is being said; we pay our taxes; we don't cheat, don't kill, don't commit incest, don't do things to others that we would not wish done to us. Some of these good principles can be found in holy books, but buried alongside much else that no decent person would wish to follow: and the holy books do not supply any rules for distinguishing the good principles from the bad.

One way to express our consensual ethics is as a 'New Ten Commandments'. Various individuals and institutions have attempted this. What is significant is that they tend to produce rather similar results to each other, and what they produce is characteristic of the times in which they happen to live. Here is one set of 'New Ten Commandments' from today, which I happened to find on an atheist website.¹⁰³