In my work as president of the National Secular Society I sometimes receive manuscripts from people who have come up with what they imagine is the definitive refutation of Christian claims. "Publish this," they say, "and Christianity will end within a year!" (See here for an example.)

I find these turgid tomes no more convincing than the ones that they seek to refute. They are anti-theology, and given that theology is drivel, efforts to unpick it are hopeless.

What is theology? I think one of the best definitions was given by the sci-fi writer Robert A Heinlein when he said: "Theology ... is searching in a dark cellar at midnight for a black cat that isn't there. Theologians can persuade themselves of anything."

And, indeed they can. They can twist the language, invert the meaning of words, tie themselves into logical knots and then get admired for it. We are told theologians are there to make sense of The Big Questions.

But I have a problem with The Big Questions – you know the sort of thing: Why are we here? What comes after death? What, indeed, is the meaning of life? My problem is that these questions don't have an answer – no matter how long you think about them and however much you try to bring God into the equation, you'll get nowhere. Or, as Gertrude Stein so eloquently put it: "The answer is: there is no answer."

Take Rowan Williams, for example, who is lauded far and wide for the vastness of his theological knowledge. He is said to have a brain the size of Jupiter because he can produce convoluted writing that nobody with their feet in reality can comprehend. And because no one can fathom it, it must be very important, right? He's much cleverer than we are because he can say things that we don't understand. For instance:

"The word of God is not bound. God speaks, and the world is made; God speaks and the world is remade by the word incarnate. And our human speaking struggles to keep up. We need, not human words that will decisively capture what the word of God has done and is doing, but words that will show us how much time we have to take in fathoming this reality, helping us turn and move and see, from what may be infinitesimally different perspectives, the patterns of light and shadow in a world where the word's light has been made manifest. It is no accident that the gospel which most unequivocally identifies Jesus as the word made flesh is the gospel most characterised by this same circling, hovering, recapitulatory style, as if nothing in human language could ever be a 'last' word."

But when he has reached the very depths of his profundity what does it amount to? I can do no better than HL Mencken, who said: "For centuries, theologians have been explaining the unknowable in terms of the-not-worth-knowing."

Theology is an excuse for grown men to spend their lives trying to convince themselves, and others, that ridiculous fairy tales are true. Some of them get paid for it. On my Sky Box there are dozens of channels under the heading "Religion". If you choose one of these channels at random you will either find someone wanting your credit card details or someone strolling around a stage carrying a large Bible before him. He will be explaining to his attentive audience the meaning of some of the more ambiguous verses in the good book.

Five minutes after tuning in to such a session, you will begin to wonder whether you've had one of those strokes that make your native language incomprehensible to you. You recognise individual words as English, but they have no meaning. Despite the shouting and the emphasis put on them by the speaker, you have no idea what he is talking about. And yet the people in the audience are nodding sagely, making notes and generally seem to understand what is being said. This is theology.

I look at it this way. If science disappeared from human memory, we would soon be living in caves again. If theology disappeared from human memory, no one would notice. Theology is a completely and utterly useless pursuit. It is self-indulgence of the first order. It grieves me that public money is spent on theological colleges while real education struggles to gain the funds it needs to maintain itself.

If you wish to hear a really brilliant theologian at work, here's a great one.