How Constantine's act of faith shaped the West

*****

YORKSHIRE MUSEUM, YORK

YORK may be a fine city, but you can't imagine it was ever the pivot on which the history of the west once turned. Nevertheless, that is the thesis of a major exhibition there dedicated to Constantine the Great.

It is 1700 years since Constantine was proclaimed emperor in York, on 25 July 306. He had been with his father, the Emperor Constantius, on an expedition against the Picts, and it is satisfying to remember they were a constant nuisance to the empire and the Caledonian frontier remained the most strongly defended in the empire.

Constantius died in York and there Constantine was proclaimed his successor. It was a time when the empire was ruled by several emperors, who were often rivals. Constantine successfully disposed of the others to become sole emperor. The decisive battle was fought in 312 against his co-emperor Maxentius. Before the battle, Constantine saw a vision of the cross in the sky. When he won he took it for a sign and became a Christian.

Even so, all this is surely far too remote for him to be of interest now? Isn't this just an obscure anniversary, York's 15 minutes of fame dug up to catch more tourists? The answer is that Constantine is not only of interest, he is topical.

The exhibition includes major loans and there are items of real beauty; jewellery, coins, sculpture, mosaics, Roman painting, even fourth-century textiles of extraordinary rarity. A great cameo eight inches across shows Constantine and his family riding in a chariot drawn by centaurs. It is a fittingly grand image for the most successful emperor since Augustus. But these objects also show the rise of Christian imagery in Roman art. There are wonderful British examples here, too; a marvellous mosaic of Christ from the English villa of Hinton St Mary, for example, and a fragment of wall painting with St Peter and two Apostles from Dorset.

It is because of images like these that Constantine's proclamation still really matters today. He made Christianity the religion of the Empire. As the Christian heritage of the West is questioned by multiculturalism or attacked from outside, the way it became established and how it was shaped really is topical.

The empire in the west survived Constantine by most of two centuries, but its eventual collapse is vividly illustrated by one of the star exhibits here, the Traprain treasure. A hoard of silver buried in the fifth century on Traprain Law in East Lothian that bears dramatic witness to the wealth of the late Roman Empire, but also to its decline.

A beautiful silver plate is crudely hacked into ragged chunks of precious metal. The value of the precious metal plainly superseded the plate's artistic merit.

One of the most important items is a jug decorated with Christian scenes. By the time the jug was buried, Christianity had become the dominant religion. By allying it with the might of the Roman state, Constantine gave the church political power, power it retained long after the fall of the empire. Indeed, the empire itself might have been all but forgotten if the church that survived it had not given enduring importance to its memory. The Christian community of the West was made in the image of the Empire. It remained intact until the Reformation 1200 years later and it is striking that even then the spread of the reformed religion matched pretty closely the map of Europe beyond the boundaries of the empire. Constantine cast a very long shadow.

He also shaped the practice of Christianity. He presided at the Council of Nicea when the Nicene Creed was agreed as a formulation of the Christian faith. It is still recited today in words he approved. He built the great churches of Rome, the first St Peter's, St John Lateran and St Paul without the Walls. He built the church of the Holy Sepulchre in Jerusalem. He founded Constantinople as a Christian city and made it his capital, creating the Byzantine Empire. He founded St Sophia there, though the church was rebuilt by his successor, Justinian, two centuries later.

Of his Roman churches, perhaps St Paul's, though largely rebuilt after fire, looks most as it did 1700 years ago. It is huge and the impact as you enter is breathtaking. There may have been something a bit like it in York, too.

One of the exhibits here, found in the city, is a huge capital from a column that must once have stood in a building fit for an emperor. There is a bust of Constantine himself, also found in York, which must have adorned such a place. Two massive Roman towers in the city wall hint at the scale of the building of which they were originally part.

Constantine did not forget York. One of the most remarkable exhibits is a contemporary copy of a letter he wrote to the Palestinians commanding tolerance of Christians (does nothing change?). In it he recalls almost sentimentally his imperial origins in Britain.

Constantine did not just demand toleration for Christianity, however. He encouraged toleration in a very modern, multicultural way. The exhibition demonstrates how Christianity coexisted with other religions. You could subscribe to several options at once, it seems. The old gods were worshipped alongside the new and among them the sun was much revered as a deity. There is a relief of the face of the sun god from Hadrian's Wall that is as elegantly simple as a Matisse.

From the range of imagery here you get a vivid sense of the fluid situation from which institutional Christianity grew and how the iconography was interchangeable between different cults.

Christ has the sun's rays and the emperor himself appears with Christ's halo. Constantine's own faith was not narrowly Christian, either. He evidently appreciated this multifaceted approach and one of his broad-minded actions shapes the life of all of us still, whether we are religious or not.

He named Sunday after the sun god and declared it should always be a holiday. So as you lie in on a Sunday morning, say a word of thanks to Constantine the Great and remember York's 15 minutes of fame that changed the world.

• Until 29 October