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Hinton St Mary Mosaic (made fourth century AD), from a Roman villa in Dorset, England

I'm walking with the gods - I'm in the gallery that's got objects from the time when Britain was part of the Roman Empire , around 1,700 years ago. Here is Mars, there's Bacchus with his wine cup, Pan piping on a silver dish... and now I've arrived at what looks like another pagan god, this time in mosaic. It's a shoulder-length portrait. He's roughly life-size, clean shaven, has fair hair swept back, and he's wearing a tunic and a robe tightly wrapped around his shoulders. But which god is this? There's a clue, because this is a man with a monogram. Behind his head are the two Greek letters 'chi' and 'rho' and that tells me at once who he is, because they're the first two letters of the word Christos, and this is in fact Christ. It's one of the earliest images we have of him anywhere, and it's an astonishing survival - made not for a church in the eastern Mediterranean or in Imperial Rome, but for the floor of a villa in Dorset, somewhere around the year 350.

"I think this is an experiment. It's an experiment in how to depict Christ in familiar iconographic form." (Dame Averil Cameron)

"I think I find it a rather distant object - the image of Christ is not, I think, an attractive one; there's that 'Desperate Dan' chin." (Eamon Duffy)

"I like the way that the eyes of the face are so... not quite staring, but they are very piercing." (Dame Averil Cameron)

"I can't imagine anybody nominating it as their favourite image of Christ." (Eamon Duffy)

This week we're looking at how, about 1,700 years ago, a number of great religions began to use human images to aid our prayers. But for the first two or three Christian centuries, the very idea of looking on the face of God, even of a god in human form, would have been inconceivable. Firstly because there was no record of Christ's appearance that artists could have based a likeness on, but even more because the Jewish inheritance of a god to be worshipped in spirit and in truth, but emphatically not to be represented in art, inhibited the early Christians from any such attempt. Yet now we all live in a world where the likeness of Christ is commonplace, a face that can be instantly recognised. How did we get there? The decision to try to show the face of Christ - probably taken because the Roman elite were so used to seeing their gods in statues, paintings and mosaics - was both a major theological step and one of the decisive turning-points in European visual culture.

This face of Christ from Dorset was made in the last century of Roman rule in Britain, which was in many ways a golden age. It was a lavish world, in which the ruling class could spend enormous sums of money, decorating their villas and putting their wealth on display in the form of spectacular tableware. In the cases around us you can see the hoards of silver vessels, spoons and even pepper pots, like the ones I talked about last week, that show a society that seems to have accommodated itself comfortably to both paganism and Christianity. The great silver dish found at Mildenhall in Suffolk shows Bacchus drunkenly cavorting with pliant nymphs, while the spoons found in the same hoard carry Christian symbols. A pagan dish with Christian spoons - that pretty well sums up Britain at this period, and it wouldn't have disconcerted anybody at the time. In Britain of the third and fourth century, Christ is merely one god among many others.

The floor was mostly made of local Dorset materials - black, red and yellowish stones, all of them set in that greatest of Roman building inventions, cement. As you went into the room, the first thing you'd have seen on the floor was a roundel, with the mythical hero Bellerophon riding the flying horse Pegasus and overcoming the Chimaera, a monster combining a lion, a goat and a serpent. It was a popular image in the Roman world, the hero zapping the forces of evil. But at the far end of the room, facing in the other direction, was another roundel. In earlier times in this sort of position you'd have expected to find either Orpheus, charming the world with his music, or the universally popular wine god Bacchus. But here we find Christ.

The pairing of Christ with Bellerophon is not as incongruous as it might initially seem to us. Here's historian Eamon Duffy:

"I think, as a historian, what impresses me about it is the juxtaposition of powerful imagery from pagan mythology - the whole story of Bellerophon, Pegasus, the Chimaera - and the way in which Christianity adapts that material for its own purposes to convey the message of resurrection, of the triumph of life over death. And the implicit comparison of Christ's work on the cross to a hero slaying a monster; that paradox that the defeat of the founder of Christianity is actually a heroic victory.

"So Bellerophon's a figure of life triumphing over the powers of darkness. Eventually that kind of symbolic imagery would find its own Christian versions in people like St George killing the dragon, or St Michael the archangel fighting the devil. But at this stage it seemed natural to adopt the pagan story, the myth which people would know from plays, from poetry, from pagan imagery."

I wonder how many of the people that crossed this floor realised that they were in fact walking from one world to another, from the familiar realm of myth to the new modern world of faith. Everybody would recognise the energetic Bellerophon. They'd be less sure who was represented by the still figure facing away from them on the other side of the room. Because very few of them would ever before have seen Christ represented. Now how do you represent a god that you have never seen? There was nothing to go on - no likeness, no model, no description of what Christ looked like. It's a testing conundrum, both theologically and artistically, and I think we can all sympathise with the Dorset artist who had to confront it. Orpheus and Bacchus would have been easy in comparison. Orpheus would be wistful, young, artistic looking, Bacchus, energetic and sexy, clearly ready for a good time. And both of these would be recognisable by their attributes; Orpheus would have his lyre, Bacchus a bunch of grapes or something similar. What is the physical attribute that Jesus would hold?

At this point few people would have wanted to show the victorious, all-powerful Christ with that shameful instrument of suffering, the cross. He had told his disciples that he was the way, the truth and the life, but it's very difficult to show any of these physically. He had announced that he was the light of the world, but it's really hard to show light in a mosaic, especially if, as here, you're dealing with a not very good artist. The mosaicist at Hinton St Mary, instead of a symbol, gave him a monogram, the 'Chi Rho' - those two letters of the Greek alphabet that begin Christ's name in Greek, written as though they were X and P in our alphabet. In our mosaic, they lie like a halo behind Christ's head. It was the symbol adopted by the Roman emperor Constantine, after his conversion to Christianity in the year 313.

Our floor was almost certainly made about 40 years later. We can be pretty confident of that, because both Christ and Bellerophon wear their hair in the fashion of about 350. It was Constantine's conversion at the battle of the Milvian Bridge that in fact made our floor possible. Before he converted, no villa owner would have dared display their Christian faith so brazenly - practising Christians had been persecuted. But now, everything was different. Professor Dame Averil Cameron of Oxford University explains:

"The Emperor Constantine seems to have converted himself to Christianity, and started to favour the Christians. He is supposed to have seen a vision of a cross in the sky some time before the battle, and thereafter he never deviated from giving privileges to Christians, which was a complete overturning of what had been happening when Christians had not even been legal. And what he did was to give tax privileges to Christian priests, to intervene in Christian disputes, to declare Christianity a legal religion, to give money to Christian churches, to start building Christian churches. So all of those actions together gave a great sort of fillip to Christianity."

And it was this fillip that must have given the owner of our villa the confidence to show us Christ looking out at us, full face, unequivocally a man of power. He wears the rich robes and the stylish hairdo that might well have been sported by the villa owner himself, but this is no local ruler and indeed no local god. The Chi Rho monogram makes it clear that what we're being shown is Jesus Christ. And there's a further clue to this man's true nature. On either side of Christ's head the artist has put pomegranates. Now, to any educated visitor, this would recall at once the myth of Persephone, carried off to the underworld, rescued by her mother, and brought back to the land of the living as a great allegory of the cycle of the seasons, of death and rebirth, of descent into hell and return to the light. By the inclusion of this simple fruit, the artist links Jesus to the pagan gods who'd also been gods of dying and returning; to Orpheus, who went to the underworld in search of Eurydice and returned, and to Bacchus who was similarly associated with resurrection.

This Dorset Christ pulls together all the hopes of the ancient world, the deepest of all human hopes: that death is only part of a larger story that will culminate in abundance of life and even greater fruitfulness.

What kind of room was this mosaic in? We just don't know. In grand Roman villas the room with the best mosaic was usually the dining room, but in this case that seems unlikely. There was no under-floor heating in this room and it faced north, so it would have been far too cold for Dorset dining. Normally the walls, as well as the floor, would indicate a room's purpose, but the walls of this room are long gone. There's one intriguing possibility: the figure of Christ faces east, and there would be just enough space for an altar between it and the wall. So this room might have been an early house church.

People have often worried about the idea of Christ being shown on a floor, and eventually this worried the Romans too. In 427 the emperor specifically banned the making of images of Christ on mosaic floors, and he ordered all existing ones to be removed. But by the date of this proclamation Britain had of course ceased to be a part of the Roman Empire. The villa at Hinton St Mary had probably long been abandoned, and so its floor remained untouched. On the whole the withdrawal of Roman power spelt cultural catastrophe, but in this instance we should be grateful, because it's that which allowed this astonishing survival.

So far this week we've been looking at religions that now have followers all round the world. Tomorrow we'll be looking at a faith that died, and at one local god who was swept away by the great globalising religions. We'll be in Yemen... with the bronze hand of Wahab Ta'lab.