MY TROUBLE with Christianity is that it is only true backwards. To take an example, here is a couplet from George Herbert: "Sin is that Press and Vice, which forceth pain To hunt his cruel food through every vein," and when I read it, two things happen almost at once.

The first is a stunned, visceral assent: a delight in the thought and its expression, and most of all in the way they are so perfectly united. The second is to note that it is not true. Pain is not always - or even often - the consequence of sin. My friend with the brain tumour and her husband are not being punished for anything anyone has done.

But suppose we read the couplet backwards - not as a description of the workings of sin in the world, but as a statement about the meaning of the word "sin", and about whatever it is that "forceth pain to hunt his cruel food through every vein".

Perhaps this is a distinction without a difference, but I think not. To read the couplet the first way is to treat "sin" as an almost scientific term: it becomes part of the chain of cause and effect, a name we give to an observable, predictable, and, in principle, even measurable pattern of events in the world. It becomes an explanatory hypothesis.

Used in this sense, we can postulate "sin" rather as we postulate the Higgs boson, and then go to see if it helps us to understand the world a bit better. But, in that sense, "sin" clearly does not exist. It is an epicycle, a meme, a failed would-be explanatory mechanism.

Read backwards, however, the couplet tells us something about the meaning of the word "sin". This is more interesting. There is a "Press and Vice, which forceth suffering through every vein" - we know this, because we see people and animals tortured all around us, usually by disease, but sometimes by deliberate wanton act.

This is clearly something that has evolved, in the sense that the earth was once lifeless, and for billions of years without conscious life or feeling. So there is something in the way the universe is which has produced the capacity to suffer, and maintained it and refined it through innumerable generations. Calling that something "sin" illuminates what the word might mean. It gives the doctrine of "Original Sin" something real to refer to, and makes it worth thinking about.

THINKING about doctrine in this way is not a habit that I am ever going to kick. I have done it almost as far back as I can remember. Perhaps the most shameful thing I will admit to publicly is that I won a scholarship to Marlborough on the strength of my essay in the Divinity exam.

But I remember, too, the feeling when I had finished writing: that I had no idea at all whether any of it was true. It was just a rhetorical exercise, in a mode in which I happen to be naturally gifted. So I concluded that the man who marked it so highly must be bluffing, too.

Subsequent, banal experiences with Christians who were stupid, cruel, smug, pharisaical, and otherwise human cemented this disillusionment. I could read the Prayer Book, and love it, but, when I attempted the Bible, I would recoil, simply unable to believe that anyone would take it as the word of God. When people describe themselves as "Bible-believing Christians", I can attach no meaning to the words, except as a label: it is like being "flag-believing Britons". Similarly, I do not know what itcould possibly mean to believe in a Creator.

None of this inoculated my imagination. I have had numerous experiences that would count as conversion, if they had actually converted me. I remember Robert Runcie celebrating a eucharist in Canterbury Cathedral, when it seemed quite irrelevant to ask if it was true: it was clearly something to be part of.

AT THE other end of the scale, a couple of fundamentalists who had given up their lives to working with junky prostitutes in a provincial town broke bread with a quiet prayerover a PVC tablecloth, and that worked, too. In Medjugorje, I got zapped by the Holy Spirit, and was for a while quite speechless with love for my fatuous and ignorant fellow pilgrims.

All this made me think that it did not matter whether I called myself a Christian, but the Lambeth Conference of 1998 made me resolve not to do so. It was a triumph of the bullies, of the self-important, the vain, and the thoughtlessly cruel. I may be a sinner, I thought, but I do not wish to be mistaken for a bishop.

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But the New Atheist movement made it quite clear to me that I'm not one of them, either. I'd like to believe in an Anglican afterlife where Professor Dawkins and Lord Carey share a hot tub in hell. It will be only hot, not scalding, and the vaporous burblings of their self-satisfactions will continue for eternity. No one else will hear, and they will never notice. All will be happy.

None of this is terribly satisfying. It is natural to suppose that our philosophical conclusions are the distinctive marks of our moral, and intellectual excellence, but that doesn't work for me. I know Christians who are nicer, cleverer, braver and more honest that I am. I even know some who appear to have no difficulty in believing the whole thing backwards - and not all of them are Roman Catholic intellectuals.

But I still can't do it myself. So why worry? Why not see it all as nonsense? Because really it isn't all nonsense. As a friend of mine, a former missionary, said once: "It's about the thing that is true even if Christianity isn't true."

Christian language does things that no other use of language can. I can conclude only that God has called me to be an atheist.