Aurelia languidly uncurled from her couch and joined them. ‘It’s called the Christogram. The first two letters in Christ’s name in Greek, superimposed - you see?’

‘I have seen this scrawled on temple walls. Even in Rome.’

Thalius said, ‘A relic of the days of persecution. Such symbols as this united a community under pressure.’

‘But now Christians are under pressure no more,’ Cornelius said. ‘And your Christogram has become a symbol of pride, yes?’

Aurelia said, ‘Ah, but the Christogram is more than that. Look again, Cornelius. Haven’t you seen figures rather like this in other contexts?’

Cornelius stepped back and tipped his head. ‘Do you know, I have. In Egypt, I think. It is rather like the ankh, an ancient mystic symbol - surely much more ancient than Christianity!’

Aurelia murmured, ‘As a girl I learned to write Greek. This is also rather like a sign you make when editing a passage of writing - chi-rho for chreston, which means good.’

‘One symbol with many meanings, then,’ Cornelius said.

‘But that’s intentional,’ Aurelia said. ‘You can scrawl a chi-rho on your wall; a Christian will see the Christogram, a pagan will see an Egyptian good-luck sign. It appeals to everybody and offends nobody. The Emperor’s advisers are wily to encourage it.

‘Constantine is a Christian. Everybody knows that. And he wishes to establish Christianity as the empire’s core religion. But almost everybody else of influence - like you, Cornelius! - remains pagan. Most of the army too, despite Constantine being one of its own. Constantine, and the bishops who manipulate him, is proceeding subtly, through tools such as this clever little symbol. But, like the rain beating on your tiles, Cornelius, each drop brings you pagans closer to the day when the roof falls in.’

Thalius said, ‘You seem to have thought deeply about this, madam.’

‘Emperors make the weather,’ she murmured. ‘It is best to pay them attention. Besides I am fascinated by the sheer machination of it all.’

Cornelius said, ‘Machination, yes. And there are plenty who doubt Constantine’s sincerity about his conversion in the first place. How is it even possible for a good pagan to become Christian?’

‘Oh, I believe he is sincere,’ Aurelia said. ‘And as for how he was converted, you can see it painted up here on dear Thalius’s wall.’

Cornelius looked again. ‘You mean the sunburst around Christ’s head?’

Aurelia said, ‘Constantine grew up as a protégé of Apollo. And some years ago he hailed the sun god, Sol Invictus, as his tutelary god. Some would identify Apollo with the sun, and others identify the sun with your Christ, Thalius: Jesus is sol justitiae, the sun of justice. So you see there is a progression, logical in its way, through an overlapping identity of deities, from Apollo, via the sun, to Christ. But it will be quite a challenge for the biographers to make sense of all this one day.’