STANDING on the steps of his minster, resplendent in his multicoloured cope and mitre amid a throng of clergy from all faiths, the Archbishop of York declared in ringing tones: “We give thanks for Constantine, Emperor of Rome and defender of the Christian faith!”

A golden eagle swooped in a great circle across the minster. The crowd cheered. Children blew trumpets, and a noisy procession set off through York’s streets, as the city celebrated on Tuesday the proclamation by legionaries, 1,700 years ago to the day, of the young Constantine as the new ruler of the Roman Empire.

In the blazing sun York gave thanks for the life of a man who, from this imperial outpost, became one of the greatest rulers of the ancient world and whose battlefield conversion ensured that, after 300 years of persecution, Christianity became the state religion.

The celebration began under the central tower of the vast Gothic minster, whose foundations rest on the former Roman military camp. “When the troops proclaimed Constantine as their Augustus, possibly at this very place, they cannot have known that this young leader would within a few years have made Christianity the religion of the Roman Empire,” said the Dean of York, the Very Rev Keith Jones. “Still less can they have suspected how from this date the Christian faith would leave its mark on the institutions, monuments and lifestyles of the world.”

Historians see Constantine’s conversion as opportunistic. Before the battle of Milvian Bridge near Rome, where he confronted Maxentius, his rival claimant for the throne in 312, Constantine saw in the sky a Greek chi-rho, the symbol of Christianity, with the words “By this, conquer”. He swore that, if victorious, he would adopt its faith. He won. And although he then ended the persecution of Christians, he kept his options open, allowing traditional Roman deities to continue. He sent his mother, Helen, to Palestine, where she, allegedly, discovered the True Cross. But he was not himself baptised until he was on his deathbed in AD337.

Nevetheless, Constantine crucially altered the course of Christianity, then rent by squabbles over doctrine and creed. In 325 he summoned bishops from across the empire to a council in Nicea, in modern Turkey, where after more than a month of arguing, they agreed on how Easter’s date was calculated, and on the wording of the Nicene Creed.

That creed was read in the original Greek at the commemorative service by Archbishop Gregorios of Thyateira and Great Britain, who represented all the Orthodox churches in Britain. Together with Archbishop John Sentamu, they then joined representatives of the Methodist, United Reformed and other Protestant denominations in laying garlands at the ruins of the Roman headquarters in the minster’s undercroft and outside at the foot of a Roman column found during excavations in the last century.

Though Constantine, who was born in what is modern Serbia, was hailed by Dr Sentamu as an “adopted Yorkshireman” (like himself), the Emperor’s role in the development of Christianity is not universally praised. Those churchmen who today denounce the interference of the state and secular political influence in ecclesiastical affairs date this “corruption” to the establishment of Christianity as the state religion by Constantine.

The Emperor also has his critics in the world of classical scholarship. The great German historian Theodor Mommsen regarded him as treacherous and fickle. “It is a difficult and thankless task to portray Constantine’s character,” he says, “but what one does discern is not pleasant.” He adds: “A noticeable thread of inconsistency runs through his actions, or rather a gradual deterioration. Outstanding, mediocre, poor might be called the three successive periods of his 30-year reign.” His assessment, however, must be seen through the eyes of a 19th-century, virulently anti-Catholic German.

Few in York on Tuesday were ready to hear any word of criticism of their local hero. The parade through the city had a joyous and carnival feel. Children and students dressed in festive clothes or Roman armour, paraded aloft banners with imperial Roman crests. In the gardens outside the Yorkshire Museum, currently staging a big exhibition portraying the life and times of Constantine, they re-enacted preRoman English and pagan traditions.

A young man, painted light blue to resemble the copper verdigris statue of the emperor outside the minister, was hauled in a chariot pulled by a giant model goose, a replica of one of the exhibits from Constantinople in the museum. Two giant puppet gladiators, Maxmimus and Tiberius, tried to catch children representing “Mr and Mrs Christian”. Children blew their trumpets, acrobats tumbled on the lawn, soldiers sweated in their helmets and armour and the chorus sang the moral York wants to draw from its most famous son: “I declare/ To each and everyone of you/ Change is in the air./ Remember my legacy./ Let there be liberty.”

It was, for York, a way to celebrate an event arguably the most important in Britain for the history of the known world. Constantine, who ruled for longer than any emperor except the first, Augustus, laid the foundation stone for St Peter’s in Rome, allowed each municipality in the empire to construct churches at state expense and founded Constantinople, the city that bore his name. It was there that the Roman — and Byzantine — empire went on to endure a full thousand years after the fall of Rome to the barbarians.

Without York, there would have been no Constantine. And without Constantine Christianity might have remained for ever a small and obscure minority sect. Archbishop Sentamu’s thanksgiving was fully deserved.