Arguments about religion as a guide to moral behaviour often go like this.

Against: Religious people do bad things. They persecute unbelievers, they torture, they bomb, they murder.

For: Atheists also do these things. Most of the carnage of the twentieth century was caused by atheists like Hitler and Stalin.

Against: Yes, but they didn't do those things because they were atheists, they did them because they were Nazis, Leninists, Maoists or whatever.

For: That's also true of religious crimes though. They are usually about political power, and religion is just an excuse. When it doesn't give a convenient excuse for a political act, religion is ignored - for instance, William of Orange fought the Battle of the Boyne with the blessing of the Pope, and the French often allied themselves with the Muslim Ottoman Empire for reasons of political advantage.

You can bat this back and forwards for hours, finally collapsing spent on to the sofa with a sugar-based treat. I would point out the following though.

I'm British, so my ancestors were probably involved in the wars of the English Revolution. If you are too, then most likely so did yours. They may have fought for the divine right of kings, or they may have fought for the autonomy of Parliament. They may have wanted freedom of trade, or they may have been more concerned about taxes. Statistically, it's likely that many of them fought on both sides.

Just for the sake of argument, let's pretend yours all fought on one side, and mine all fought on the other. It doesn't matter. We'll never know, and wouldn't much care if we did.

But in the north of Ireland, everyone knows. This is because the Catholics were on one side, and the Protestants on the other. The victorious Protestants passed down their triumphalist metaphysics through Lodges and business organisation, while defeated Catholics put their children in separate schools, and taught them never to forget.

In practice, everyone in northern Ireland probably has ancestors on both sides as well, but their metaphysics obliges them to pretend history is less complex than it actually is.

These days, the antagonism between the two sides has more to do with jobs, housing or the police. The point is, though, religion does have a way of burdening today's social divisions with the ideological baggage of yesterday's.

Incidentally, the jury is out over whether Hitler was or wasn't an atheist. In time honoured religious fashion, he made many statements on the subject, most of them contradictory.