One twist is that on Discworld, there is no lack of gods; however, few of them have any great significance: There are billions of gods in the world. They swarm as thick as herring roe. Most of them are too small to see and never get worshipped, at least by anything bigger than bacteria, who never say their prayers and don't demand much in the way of miracles.

They are the small gods, the spirits of places where two ant trails cross, the gods of microclimates down between the grass roots. And most of them stay that way.

Because what they lack is belief.

Small Gods is the story of one rather larger god, the Great God Om, who manifests himself to a novice monk called Brutha, in the Citadel at the heart of the city of Kom in the lands between the deserts of Klatch and the jungles of Howondaland.

Brutha's attitude to religion is a very personal one. He runs his own life by it. In contrast, Deacon Vorbis believes that the role of religion is to run everybody else's life. Vorbis is head of the Quisition, whose role is 'to do all those things that needed to be done and which other people would rather not do'. Nobody ever interrupts Vorbis to ask what he is thinking about, because they are scared stiff that the answer will be 'You'.

The Great God's manifestation takes the form of a small tortoise. Brutha finds this hard to believe: I've seen the Great God Om ... and he isn't tortoise-shaped. He comes as an eagle, or a lion, or a mighty bull. There's a statue in the Great Temple. It's seven cubits high. It's got bronze on it and everything. It's trampling infidels. You can't trample infidels when you're a tortoise.

Om's power has waned because of a lack of belief. He tests his strength by silently cursing a beetle, but it makes no difference and the insect plods away unperturbed. He curses a melon unto the eighth generation, but with no evident effect. He inflicts a plague of boils on it, but all it does is sit there, slowly ripening. He vows that when he returns to his rightful state, the Tribes of Beetle and Melons will regret not responding. For on Discworld, the size of a god is determined by the strength, and amount, of belief in him (or her, or it). Om's church had become so corrupt and powerful that the fearful belief of the common people had been transferred to the church itself -it's very easy to believe in a red-hot poker -and only Brutha, simple soul, still truly believes. No god ever dies, because there is always some tiny pocket of belief remaining somewhere in the world, but a tortoise is pretty much as low as you can get.

Brutha is going to become the Eighth Prophet of Om. (His grandmother would have made it two generations before, but she was a woman, and narrative imperative forbids female prophets.)

Vorbis's job is to ensure that all Omnians remain true to the teachings of the Great God Om, which is to say, they do what Vorbis tells them. The presence on the Discworld of the god itself, causing changes to all the old teachings and generally making trouble, is not greatly to Vorbis's taste. Neither is the presence of a genuine prophet. Vorbis is faced with the inquisitor's spiritual dilemma, and resolves it in the time-honoured manner of the Spanish Inquisition (which, basically, is to tell oneself that torturing people is fine because it's for their own good, in the long run).

Brutha has a much simpler vision of Omnianism: it is something for individuals to live by.

Vorbis shows Brutha a new instrument that he has had made: an iron turtle upon which a man or woman can be spreadeagled, with a firebox inside. The time it takes for the iron to heat up will give them plenty of time to reflect on their heresies. In a flash of prophecy, Brutha realises that its first victim will be himself. And in due course, he finds himself chained to it, and uncomfortably warm, with Vorbis watching over him, gloating. Then the Great God Om intervenes, dropped from the talons of an eagle.

One or two people, who had been watching Vorbis closely, said later that there was just time for his expression to change before two pounds of tortoise, travelling at three metres per second, hit him between the eyes.

It was a revelation.

And that does something to people watching. For a start, they believe with all their heart.

The Great God Om now is truly great. He rises over the Temple, a billowing cloud shaped like eagle-headed men, bulls, golden horns, all tangled and fused into one another. Four bolts of fire whir out of the cloud and burst the chains that fastened Brutha to the iron turtle. The Great God declares Brutha to be Prophet of Prophets.

The Great God gives Brutha the opportunity to make some Commandments. The Prophet declines, having decided that 'You should do things because they're right. Not because gods say so. They might say something different another time'. And he tells Om that there will be no Commandments unless the god agrees to obey them, too.

Which is a new thought, for a god. Small Gods has many wise words to say about religion and belief, and it makes the point that in their own terms the Inquisitors believe they are doing good.

Fyodor Dostoyevsky's The Brothers Karamazov has a scene in which the Grand Inquisitor encounters Christ, and explains his point of view, including why Christ's renewed message of universal love couldn't have come at a worse time and will only cause trouble. Just as the presence of Brutha, a genuine prophet, was not at all to the liking of Deacon Vorbis.

The Spanish Inquisitors' justification of their actions was philosophically convoluted. The purpose of their tortures was straightforward: it was to save a sinner from eternal damnation. The tortures of Hell would be far worse than anything that the Inquisitors could inflict in this world, and they would never stop. So of course they were justified in using any means whatsoever to save the poor soul from destruction. They therefore believed that their actions were justified, and in accordance with Christian principles. Not to act would have been to leave the person concerned in danger of the terrible fires of Hell.

Yes, but what if they were wrong in this belief? This is the convoluted bit. They weren't quite sure about their religious position. What were the rules? If they failed to convert one tortured heretic, would the Inquisitors burn forever? If they converted one heretic, would their souls be guaranteed a place in Heaven? The Inquisitors believed that by inflicting pain and terror without knowing the rules, they risked their own mortal souls. If they were wrong, it was they who would be immersed in the eternal flames. But they were willing to risk this enormous spiritual danger, to take upon themselves all of the consequences of their actions, should they turn out to be wrong. See how incredibly magnanimous they were being, even as they burned people alive and hacked them limb from limb with red-hot knives ...

Clearly something is wrong. Dostoyevsky solves his own narrative problem by having Christ respond the way his own teachings would lead him to: he kisses the Inquisitor. This is an answer, of a kind, but it doesn't satisfy our analytical instincts. There is a logical flaw in the Inquisitors'

position: what is it?

It's very simple. They have thought about what happens if their belief that their actions are justified is wrong -but only within the frame of their religion. They have not asked themselves what their position would be if their religious beliefs are false, if there is no Hell, no eternal damnation, no fire and brimstone. Then their justification would fall to bits.

Of course, if their religion is wrong, then its doctrine of brotherly love could also be wrong. It doesn't have to be: some parts might be fine, others nonsense. But to the Inquisitors it is all of one piece, it stands or falls as a whole. If they are wrong about their religion, then there is no sin, no God, and they can cheerfully torture people if they want to. It really is a nasty philosophical trap.