The humanist philosopher Simon Blackburn recounts a wonderful anecdote told to him by a colleague about a high-powered interfaith panel discussion. Each speaker took turns to explain some key ideas of their faith – Buddhist, Hindu and so on – and the response from other panel members was always along the lines of: "Wow, terrific, if that works for you that's great." The same response greeted the Catholic priest who talked of Christ and salvation, but instead of being pleased with their enthusiasm "he thumped the table and shouted: 'No! It's not a question of if it works for me! It's the true word of the living God, and if you don't believe it you're all damned to hell!'"

"And they all said, 'Wow, terrific, if that works for you that's great.'"

The puzzle for many of us is why this kind of thing doesn't happen more often. The simple fact is that almost everyone who is serious about their religion believes that others have got it badly wrong. If they're not going to hell, then they are at least missing out on life's most important truths. So why the silence about the errors of other faiths?

The most obvious explanation is simple civility and a respect for different opinions. It would be rude and arrogant for a member of one religion to criticise another, so if they can't say anything nice, they don't say anything at all. But this doesn't add up. Rowan Williams, for example, does not seem to think he's being rude or arrogant when he criticises the government (especially since he frames it as "encouraging the present government to clarify what it is aiming for"). The Dalai Lama is not considered rude or arrogant for criticising capitalism for being "concerned only with gain and profitability".

The Association of British Muslims was not rude or arrogant when it quite rightly criticised the UN general assembly for removing a clause abut the sexual orientation of the victims from its resolution on extrajudicial, summary or arbitrary executions. It seems religious leaders have no problem finding civil ways of being critical of everyone apart from each other.

So there's got to be something else going on here and it doesn't seem uncharitable to suggest that it's a kind of sticking together for self-interest, a version of "the enemy of my enemy is my friend". A religion's direct competitors are not the biggest threat. People rarely switch between them and because the traffic tends to be two-way, the net affect is usually negligible anyway. The real danger comes from people giving up on religion altogether. So religions have an interest in "sector building", seeing promotion of the profile of their kind existential product as being more important than their particular brand.

It's another symptom of what Daniel Dennett calls "belief in belief". Sure, people do sincerely believe the specific tenets of their faith, albeit with varying degrees of intensity and selectivity. But whereas the exact contents of the creed are up for negotiation, that there must be one is not. What matters above all else is to be religious: how exactly you do so is mere detail.

Of course, this isn't how people explicitly or consciously see it. But if we judge people according to what they do rather than what they say, this explanation makes most sense to me. And I think there could be some benefits if religious people were to acknowledge this.

First of all, it would provide an opportunity to question whether the tactical alliance is really the right one. If "the enemy of my enemy is my friend" then I think many people are confused about who their friends and enemies are. There are plenty of moderate Christians, for example, who have much more in common with sympathetic atheists than they do evangelicals. Gay Christians should be more critical of their homophobic co-religionists than of atheist materialists. Such people should leave the opportunistic coalition of faith and join a principled coalition of the reasonable.

Second, recognising that belief in belief matters more than belief is a way of moving religion more in the direction of practice and form of life, away from discredited supernatural creeds. As I've said more than once in this series, I'm all in favour of religion being more about practice than belief, but it is just wishful thinking to believe it already is.

However, I am not holding my breath waiting for either development to happen. The more depressing truth seems to be that for all their aspirations for transcendent truth and higher purpose, religions behave like any other worldly individual or organisation and end up doing what protects their secular interests, not what most aligns with their values. And I'd be a liar if I said: "Wow, terrific, if that works for you that's great."

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