Nothing captures the commercialisation of Christmas quite as effectively as the history of Santa Claus. To illustrate the point, look at a wonderful 16th-century painting that hangs in Room 7 of the National Gallery in London. Most people who pass by it are unaware of its significance. But this panel, circa 1555-60, preserves a particularly pure element of the Christmas spirit.

The painting, attributed to the Tuscan mannerist Girolamo Macchietti, depicts the most important legend of St Nicholas of Myra.

To the right, a nobleman slumbers, surrounded by his three daughters, who are also asleep. According to tradition, the family was so poor that the father was on the brink of selling his girls into prostitution. To the left, their saviour appears at the window, dressed in a sumptuous orange tunic adorned with a red robe. Under the cover of darkness, he prepares to lob through the aperture the second of three balls of gold (each represents a purse stuffed with money) that will provide dowries for all of the daughters, so that they won’t have to sell their bodies to survive.

For Macchietti’s contemporaries, this youth would have been instantly recognisable as St Nicholas. But, today, he goes by a much more familiar name: Father Christmas.

This may come as a surprise. How can Macchietti’s Mr Goldenballs, with his gilt sandals and curly, glowing hair, be related to roly-poly, red-faced Santa Claus? For starters, he’s too thin. And beardless. He is dressed like an inhabitant of the Mediterranean, not Lapland. He is standing by the window, not peering down the chimney. And, anyway, where’s his retinue of reindeer?

But, then, this is one of the sad truths at the heart of Christmas present. These days, it isn’t only the birth of Christ that is threatened with oblivion. The charitable St Nicholas, associated with Christmas since time immemorial, is rapidly sliding towards anonymity, too. Meanwhile, the stock of his more recent incarnation as Santa Claus, the darling of department-store managers, filmmakers and advertising copy-writers the world over, continues to rise.

Canon James Rosenthal, who has earned a reputation as one of the Church’s leading authorities on St Nicholas, believes it’s time that the “real” Father Christmas is remembered.

“I always think it’s sad that people are ignorant of the origins of our customs,” he says. “Santa Claus is fine, but St Nicholas is so much better. Like us, he is real.

“I believe there is a bit of St Nicholas in all of us. For Christians, he is a model to push chubby Santa back into fairyland.”

St Nicholas’s standing is currently so low that Canon Rosenthal was recently banned from visiting children held at an immigration centre in Bedfordshire. When he arrived at Yarl’s Wood, dressed as St Nicholas, wearing a magnificent fake white beard and a bishop’s mitre, he had hoped to deliver presents donated by the congregations of several London churches. But he was turned away by security guards, who eventually called the police. “I felt like a criminal for trying to spread cheer and a few gifts,” Canon Rosenthal told me this week.

St Nick must have been a pretty impressive figure to inspire such devotion, but, in truth, few facts about him are known. He was probably born around AD 260 in the port of Patara on the southern coast of what was then Asia Minor, now Turkey. He grew up in the eastern reaches of the Roman Empire, which was still hostile to Christianity, but found himself drawn to the new religion, and rose to become the bishop of Myra, now the Lycian town of Demre. He died in Myra in 343, possibly on December 6 (the date on which he is usually venerated today).

“The first Life of St Nicholas is from the 9th century,” says Robin Cormack, professor emeritus of the Courtauld Institute of Art, and an expert on Byzantium. “Maybe St Nicholas was a bishop in the 4th century AD; all else seems fiction.”

But what fiction! Scintillating legends quickly gathered around the memory of Myra’s bishop, who acquired a reputation for generosity. Aside from rescuing the daughters of the impoverished nobleman in Macchietti’s picture, he is said to have resurrected three boys who had been killed by a psychotic butcher, who’d chopped them up, salted their remains in a barrel, and planned to sell their cured body parts as ham during a period of famine.

Over the centuries, St Nicholas evolved into the patron saint of sailors and fishermen, pawnbrokers, children, scholars, druggists – and even people being mugged.

By the 10th century, a basilica containing his relics had been built at Myra. In those days, the remains of holy figures were big business, since thousands of pilgrims flocked to shrines all over Europe. In 1087, a bunch of brigands from the Italian port of Bari on the Adriatic Sea set sail for Myra, where they looted the basilica, before returning home with the exhumed remains of St Nick. A shrine was quickly established back in Italy, and people flocked to Bari to peddle the holy “manna” which was said to drip from the saint’s bones.

The arrival of St Nicholas in Italy accounts for his popularity among Italian artists of the Renaissance. Fra Angelico and Masaccio both depicted the saint in altarpieces. Veronese painted the saint, with a white beard, in a grand canvas from 1562 that can be seen in the National Gallery.

St Nicholas was soon venerated across Europe. He was especially beloved in Holland, where, to this day, children receive gifts on the Feast of St Nicholas rather than Christmas Day. The tangerines traditionally left as gifts in the stockings of children who have been good allude to St Nicholas’s emblem – three balls of gold.

His transformation into Father Christmas only occurred after the Dutch had emigrated to North America in the 17th century. In the New World, they continued to observe the feast day of Sinterklaas, as they called St Nicholas. This dialectical quirk became “Santa Claus”.

Most of Santa Claus’s current iconography – the flowing beard, red-and-white livery, reindeer – dates from 19th-century America, where the traditions of the early Dutch settlers were fondly recalled. Clement C Moore’s poem The Night Before Christmas, published anonymously in 1823, cemented the image of Father Christmas in the popular imagination as a jolly old soul with a white beard who arrives through chimneys to deliver gifts into stockings, before riding off into the night on a sleigh laden with toys and powered by prancing reindeer.

The New York caricaturist Thomas Nast later refined our image of Father Christmas, fattening him up in a series of cartoons that appeared in Harper’s Weekly from 1863 onwards. Nast was also responsible for changing the colour of Santa’s cloak from tan or green to red, decades before the Coca-Cola advertising campaigns of the mid-20th century, featuring Swedish artist Haddon Sundblom’s vision of Father Christmas swigging from a bottle of Coke. The first of these ads appeared in 1931, marking a watershed in St Nicholas’s transformation from icon of Christian self-sacrifice to the plump, friendly face of yuletide capitalism.

The question remains why it was specifically St Nicholas, rather than any other saint, who became indelibly linked with Christmas. For Canon Rosenthal, the answer is simple. “St Nicholas hit all the right chords in the hearts, minds and imaginations of the people,” he says. “He went to prison for his faith, he smashed pagan altars, he gave away his wealth, and he even restored three boys to life. Not bad for one person.”

But Prof Cormack is not so sure. “No one understands the reason for the popularity of St Nicholas,” he says. “All the stories [associated with him] are conventional saints’ stuff. He just got lucky.”