We are in the middle of the most elaborate and complex remembering ritual that Europe has devised in the past 2,000 years: Easter. A four-day solemn liturgy turned into a holiday, where crucifixes jostle pagan eggs and chocolate bunnies. An eclectic, shifting remembering.  Our collective memories are welcoming places, and one image, that of Jesus, has absorbed and appropriated elements of other traditions and aspirations in order to shape our communal remembering.  There is not much we can say with absolute confidence about the early church, but we can be fairly sure that the first Christians would not have dreamed of making a likeness of Jesus. Not just because there was no record of his appearance that they could have based a likeness on, but more because their Jewish inheritance of a god, worshipped in spirit and in truth but emphatically not represented in art, would have inhibited them (as it later did the Muslims) from any such attempt. For the first two or three Christian centuries the idea of looking on the face of God, even in human form, would have been inconceivable. Yet we now all live in a world where the likeness of Christ is commonplace, a cliche repeated in films and books or borrowed for very different purposes by advertising and politics. The decision to try to show the face of Christ was not just a major theological step, but one of the decisive turning points in European visual and, indeed, political culture. We don't know where it first happened, where an artist first tried to capture the likeness of Jesus, and it is probable that many early attempts have been lost. But the strongest candidate for the oldest, securely datable image of Christ was made not for a church in the eastern Mediterranean or imperial Rome, but for the floor of a dining room in Dorset around the year 360.  The face of Christ in Dorset is now in the British Museum's Weston gallery of Roman Britain. Here, you get an extraordinary impression of what life must have been like in this northernmost province of the Roman empire. The last century of Roman rule (c300-400AD) was, in many ways, a golden age. Roman Britain was prosperous, indeed rich. The ruling class built large villas with huge estates and enormous sums of money were spent on ostentatious decoration and spectacular silver tableware. The hoards of silver found, especially in East Anglia, include bowls, plates, spoons and even pepper pots (pepper was so expensive that it was used to pay part of the ransom that Alaric later demanded from Rome). This is a society that seems to have accommodated itself comfortably to paganism and Christianity. The great silver dish found at Mildenhall in Suffolk shows Bacchus, the Roman god of wine, drunkenly cavorting with pliant nymphs in an elegant and very alcoholic dance: some of the spoons found in the same hoard carry Christian symbols.  The Dorset dining-room floor mosaic was found in the ruins of a large Roman villa at Hinton St Mary, a small village in the Blackmoor Vale about eight miles from Shaftesbury. It is made mostly of local Dorset materials of black, red or yellowish stone with pieces of ceramic here and there, all set in that greatest of Roman building inventions, cement. In the corners are representations of the four seasons, and there is the usual arrangement of garlands and decorative strips. In the centre of the floor are two roundels. In one, the mythical hero Bellerophon, riding the winged horse Pegasus, overcomes the monster Chimaera. It was a popular image in the Roman world, the hero zapping the forces of evil. In the other roundel, you would have expected to find either Orpheus charming the world with his music, or the wine god Bacchus. But, in Hinton St Mary, it is Christ. In a society that had for centuries been used to seeing its gods, someone had decided that this new god must make an appearance.  But here the artist must have had a real problem. There was no prototype, no model, no description. He had to invent Christ in order to show him. How do you represent a god that you have never seen? It is a testing conundrum, theologically and artistically. This artist must have seen and made images of Orpheus and Bacchus in similar mosaic floors. Orpheus would generally be wistful, young and artistic-looking. Bacchus would be energetic and sexy. Each would essentially be distinguished by his attributes: Orpheus would have his lyre, Bacchus a bunch of grapes or something similar. You would know who they were, not because of how they looked, but because of what they held. But this is difficult with Jesus. What is Jesus's physical attribute? He told his disciples that he was the way, the truth and the life, but it is very difficult to show any of these physically. He announced that he was the light of the world, but it is extraordinarily hard to show light in a mosaic. And although he did compare himself to a vine and his followers to branches, the vine was the property of Bacchus and could have led only to confusion.  Then there was the question of what general tone his face should carry. Clearly not specifically artistic or pleasure-seeking like Orpheus or Bacchus, but equally, for a Roman public used to seeing their gods as heroes, not looking like a poor suffering man brutally put to death on the gallows.  The artist at Hinton St Mary found an ingenious and revealing solution: he looked at a coin. At least we can be fairly certain that is what he did. He seems to have taken a coin of the emperor, or of the man who claimed to be emperor and who had usurped Britain and Gaul in the middle of the fourth century, Magnentius. The emperor is shown as you would expect, robed and severe in the circular field of the coin. On the other side, that particular emperor chose the Christian symbol of the chi rho, the two letters that begin Christ's name in Greek, written as though they were X and P in our alphabet. It was the symbol that Constantine, the first Roman emperor to become a Christian, had taken after his conversion and his victories in 313. It had become the logo of the new religion throughout the western empire. On a coin like this (and one has been found in a a grave near Hinton St Mary), Magnentius shows himself not just as emperor, but as a Christian emperor, heir to Constantine.  The artist has simply combined the front and the back of the coin. So, in the mosaic, an imperial bust looks out at us full-face, robed with authority. We know we are looking at a ruler. And to make it clear that we are looking not at a secular ruler, but at the king of kings, the artist has put the chi rho monogram behind his head. You would not recognise this as the face of Christ, for you would never have seen a picture of Christ before. But you would know this was Christ, and Christ the emperor. Jesus has found his attribute, and it is temporal power. In this, perhaps the oldest surviving image of Jesus, and certainly the oldest known in Britain, we already have the fusion - perhaps deliberate confusion - of the authority of God and the authority of Man. In mid-fourth-century England, it is no longer possible to distinguish what should be rendered unto Caesar and what should be rendered unto God, because Caesar claims God's authority on a coin and God looks like Caesar in the mosaic.  Yet the artist wanted to show more: that this man was not only lord of life but lord over death. So, he has put a pomegranate either side of Christ's head. For any Dorset diner, this would resemble the myth of Persephone carried off to the underworld by Pluto, 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 much older resurrection myths: to Persephone, and to Orpheus, who went to the underworld in search of Eurydice. In each case, the miracle was accomplished by love. This Christ pulls together the hopes of the ancient world, the deepest of all human hopes, that love is stronger than death.  The Hinton St Mary mosaic is a striking example of how memories can be manipulated to create new meanings. The association of imperial power that the coin image evokes is so strong that it overwhelms every other aspect of the life and teaching of Jesus, tying the church firmly to the apparatus of the state. It is an irony of history that it should appear in England, for it documents an important early step towards the established church. And the pomegranate associations show that this church was, even then, a broad one: there was no need to cast off the old religion completely before embracing the new. This is both/and, not either/or theology. It is the spiritual equivalent of fusion cuisine. There is no reason to suppose it was any less nourishing.  · Neil MacGregor is director of the British Museum. The Hinton St Mary Mosaic can be seen in The Museum of the Mind, Art and Memory in World Cultures at the British Museum, London WC1, until September 7. Details: 020-7323 8000. A version of this article was broadcast on Radio 4's The Human Face of God
How do you create an image of Christ when you've no idea what he looks like? Neil MacGregor believes an ancient mosaic on a dining-room floor in Dorset may provide some answers