In medieval times, Western philosophy was more or less indistinguishable from Christian theology. It had become little more than a tool for advancing theological positions to the greater glory of a God that no sane person dared to doubt the existence of.

Today, many philosophers find philosophy an equally effective tool for advancing atheism.

My book does not evangelise for God and religion or for atheism, secularism and science. It simply explores in an objective and unbiased way what philosophers have said over the centuries about the idea and nature of God, his relationship to the world and his existence or non-existence.

It concludes that agnosticism is the only tenable philosophical position. But it does not evangelise for agnosticism or advocate an agnostic church, for it is perhaps not wise to live and die as an agnostic even though, at the time of writing this, I am indeed one.

These days "agnostic" is taken simply to mean a person who is undecided or in doubt as to whether God exists or not. More precisely, it refers to a person who holds that certain knowledge of a supreme being is impossible. Thus, I am currently an agnostic in both the simple and the precise sense.

I do not buy into many of the ideas that some religious fanatics have offered me on my doorstep over the years and I do not see how it is possible for me to choose to commit myself to believe what they say if I find that, in thinking rationally about what they say, I just do not believe it.

Surely it is basic common sense that belief should not be a matter of what a person wants to believe but a matter of what reason and evidence dictate.

I was once told on my doorstep that one day the kingdom of God will be established on Earth and that the lion will lie down with the lamb. Doubting Thomas that I am, I asked what the lion would eat.

The reply was that the lion would eat straw. I may not know anything for certain but in light of all the myriad, interconnected beliefs that I hold about the everyday world on the grounds of sound empirical evidence, the claim that one day the lion will lie down with the lamb and eat straw is utter nonsense. I just do not believe it and cannot believe it and I would insult my own intelligence if I were somehow able to persuade myself to believe it.

Of course, to be fair, not all religious thinking is as silly and way-out as the example I have taken here. Far from it. Some theists are philosophically extremely sophisticated, just as some atheists are extremely obtuse.

Belief in God, and fundamentalist beliefs in the literal truth of anti-scientific, religious mumbo jumbo, may be two completely different things.

As for atheism, to be an outright atheist is to assert that one knows for sure there is no God. But I am pretty sure that nobody knows this for sure. Philosophy reveals that there is very little if anything that we can know for absolute certain.

Arguably, all claims to knowledge beyond mere truisms such as "A father is a male parent" are subject to doubt and uncertainty.

It is my scepticism that prevents me from being an atheist, from committing myself to such a strong position of certainty. A sceptic is a person who doubts and just as various philosophical doubts and problems that I am aware of prevent me from asserting the strong theistic position that God definitely exists, so various other philosophical doubts and problems prevent me from asserting the strong atheistic position that God definitely does not exist.

Some philosophers have quite sensibly argued that to find God - to acquire heartfelt belief in him - a person must set aside reason and scepticism in favour of faith and all the various picky little doubts that make reasoned belief in God so difficult, if not impossible, and instead undertake to approach life in such a way that in time he acquires genuine religious convictions.

In other words, a person must take a leap of faith in the hope that if he starts behaving as though he believes, he will eventually believe. This is interesting stuff and I certainly relate to the moral concerns and mortal anxieties of those great thinkers such as Blaise Pascal, Lev Tolstoy, Soren Kierkegaard and William James who write of the pressing need to take a leap of faith. Yet still the sceptical philosopher in me questions the whole notion of faith.

As a Jean-Paul Sartre scholar, I am constantly reminded of his view that all faith is bad faith, that faith is allowing oneself for motives of psychological comfort to be convinced when one is barely persuaded. I will also never forget what my old philosophy professor Anthony Manser once said to me as an undergraduate, blowing smoke as he pointed at me with his pipe: "Faith is believing what you know ain't true." Faith is believing - or pretending, hoping or needing to believe - what it is not possible to be sure about.

It is an interesting question in itself to ask why people are so interested in the God question. Mention God in mixed company and straightaway people dive into a debate about whether or not God exists, arguing for their respective theism, atheism or agnosticism.

Perhaps their main concern is to respond to organised religion, which tends to preach that a person is in serious trouble if he or she does not believe, if he or she does not subscribe to what religion offers.

The clear message is that to fail to believe in God is to go against God. More to the point, it is to risk missing out on eternal life or even to condemn oneself to eternal damnation in the fires of hell.

To threaten people with eternal damnation if they do not sign up to the creed has always been one of the main ways in which religious organisations recruit members and keep them obediently towing the line.

Those who believe in God feel relieved that they have settled the question on the safe side. Meanwhile, the atheists are taking a risk but refuse to be threatened. As for agnostics, many of them feel they ought to make more effort to believe, just to be on the safe side, just in case a punishing God really does exist.

I frown upon the hypocrisy and intolerance of organised religion that insists on selling the disease along with the cure - the disease being guilt and fear and the cure being absolution.

This is an edited extract from The God Confusion: Why Nobody Knows The Answer To The Ultimate Question, published by Bloomsbury and available now as an ebook at £9.47 and in hardback, price £12.99, from November 7.