But what of Christianity? If, in preserving Rome, the Church was corrupted or destroyed, Thalius concluded sadly, the loss to mankind would be greater even than if Rome fell. So what was to be done?

He closed his eyes in brief prayer, seeking guidance. If only Constantine could see the effect of his policies on his subjects, perhaps he could use consent, not force, to unite the empire around a new set of goals to meet the challenges of the age-rebuild the empire as a truly Christian nation-and all fifty, sixty million of its citizens could move forward together. A letter, he thought: yes, a letter signed by a spectrum of concerned but good-hearted individuals-a letter backed by the mysterious authority of the Prophecy-that might encourage the Emperor to clarify his own thinking on many issues. Perhaps it could be circulated to other concerned groups. A petition, then. Standing there, eyes closed, he imagined how he might draft the first paragraph-he would need advice on the honorific to be used when addressing a modern emperor-

'Falling asleep, Thalius?' Cornelius's voice was sardonic.

Thalius's eyes snapped open.

Aurelia was watching him, her face impassive. 'What are you thinking, Thalius?'

Irritated, defensive, he said, 'I am thinking that even Constantine's actions are trivial compared to the greater forces that shape our age. I am thinking that perhaps we are simply distracting ourselves from the uncertainty of the future with a word game.'

'Perhaps that's so,' Cornelius said, apparently not offended. 'And perhaps we aren't as shallow as you seem to believe, Thalius. Whatever you say we are faced with a decision. What will you do?'

Thalius, embarrassed by his outburst, took a deep breath. 'I am with you.' He glanced at Aurelia. 'Read the Prophecy.'

Aurelia stepped towards the boy, who, with the dogged, choice-free patience of a born slave, continued to wait, back bent, tunic pulled over his head. With surprising tenderness Aurelia touched his shoulder. 'I'm sorry, child. It will be over in a moment.' And with one manicured fingertip she began to trace the acrostic.

'From A to O, alpha to omega-bottom left to upper right, for by your tradition God is always to be found on the right hand side, am I correct, Thalius? This is a path to God, then, the true route for the pious. But it is a long and tangled path. How do we proceed? I believe these diagonal letters are a clue: C, O, and then up to the N…C-O-N referring to Constantine perhaps? And then I suppose we follow the diagonal back down again-S, T-and then to the corner-A, and work our way back up the long diagonal…'

Thalius held his breath as the trail of her finger, working back and forth along the diagonals of the square, picked out words:

'From alpha to omega,' he read. 'To stand firm. To die.'

Cornelius straightened up and snorted. 'Is that it? It's not even a sentence. A nice motto for you pious types, I suppose. Hold true in death and you will be led to God. Fine. But it's no use to us, is it?'

Aurelia said, 'Look again, you fool. There are layers of meaning. Can you not see it? Constare-Constantine-perire…'

And in that moment Thalius saw the meaning of the message. The scrap of text was three hundred years old, yet it was quite specific, and it went to the heart of his own modern dilemma like an arrow from a bow.

If the true Church was to survive, Constantine had to die.

The slave boy was beginning to tremble.

After Audax was sold to Thalius, he had been brought all the way across the country to the coast at a place called Rutupiae. Then he was taken through an immense city and across a huge river, through mile after mile of a green land of farms and canals and ditches, and at last to another city, Camulodunum. Now he was bundled into Thalius's cart once again to be hauled off to what Tarcho called 'the Wall'.

Tarcho tried to explain where he was going, with maps sketched in the dirt with sticks. Audax didn't understand what a map was in the first place, and north, south, east and west were all the same to him.

And he didn't want to leave Thalius's house in Camulodunum, because of the food. He had been fed in the kitchen, with the slaves and servants, and sometimes Tarcho joined him. It was better food than he had had in his life. Sometimes he was given so much that he stopped being hungry, so much food he couldn't finish it. Tarcho promised that Audax would never go hungry, he could always share Tarcho's bread.

But whether he understood or not, of course, whether he wanted to go or not, he had no choice about making the journey.

And now they had new people to travel with, and that was another problem for Audax. The lady Aurelia rode with them, and sometimes Ulpius Cornelius too. The three of them, Thalius, Cornelius and Aurelia, would huddle in the back of the cart, whispering.

Audax, utterly dependent on their goodwill, was acutely sensitive to their moods. Thalius, overweight, fussy, clumsy, was a good man. Audax couldn't imagine him harming anybody on purpose. But he was vague. When he turned his attention on you, you could bask in his kindness, but then he would turn away, his head full of thinking, and he would forget you even existed. Thalius was all right, but he wasn't to be relied on.

As for Aurelia, she was an old woman with the body of a girl. Caked in creams, she trailed a cloud of stinks that made Audax's nose itch. She hadn't been unkind to him, that time when she had touched the tattoo on his back. But to her Audax was just a slave, no more than a bit of furniture, and just as easily disposed of. Audax understood this very well.

It was an attitude Ulpius Cornelius shared too. But sometimes Cornelius looked at Audax with a searching stare. Perhaps Cornelius was a 'dirty man', as the boys in the mine had called the men, slaves and overseers alike, who had used them. Perhaps he was working out how he could get Audax alone, or dreaming of what he would do if he could. But he made no approach to Audax. Tarcho was careful not to let Audax out of his sight.

All this discomfort was dwarfed by a deeper dread.

Audax had spent almost all his life in the mines, shut up in the dark. Before Thalius and Tarcho came he had had only broken memories of the wider world, relics of when he was very small. Now he was stuck out in the open, and he hated the vast pulsing of day and night. It seemed unnatural, somehow out of control.

Thalius ambitiously tried to explain to Audax the difference between 'finitude' and 'infinity'. Audax's deep confusion came from a life spent in the enclosed and finite, and now he was stranded in a world of openness without end. Audax dimly grasped these ideas. But he thought it just went to show that Thalius had never been a slave. Slaves understood infinity, even if they had no words for it, for slaves faced a lifetime of labour, of an utter lack of choice, without end. Servitude was infinity.

The one element in this huge open world of the outside that he felt drawn to was the sun. When the sky was clear the warmth of that great lamp in the sky sank deep into his bones and drew up his blood. Thalius gently explained to him that it was the sun that gave life to all things on earth, and that some people worshipped it as a god. Some believed it was a form of Thalius's own god, the Christ, who had also been a man. The sun reminded Audax of Tarcho, in his strength, his warmth, his patience. Audax imagined Thalius's Christ as a huge bearded soldier in the sky who smelled of sour German cabbage.