HOLY Trinity Church in Sloane Street, London, was crawling with children. They sat on pews, in the aisles, on cushions, on shoulders and knees. They were gazing toward the chancel steps where there sat a saint with a mitre, a long, white beard and scarlet cope trimmed with gold. “Ho Ho Ho!” he boomed. “Who do you think I am?”

As with one voice, united in what they understood to be the liturgy of the season, they replied: “Father Christmas!”

There followed a short history of the saint, a couple of jolly hymns and a prayer or two, during which, like a miracle, the church full of children listened as silent as the night. At the end, the red-robed figure asked again: “Children, who am I?”

This time they cried out, as loud as before: “St Nicholas!”

This was as good a way as any to educate children in the “real” meaning of Christmas, and children in this part of the world, where Harrods has had a Santa in situ since November 5, are probably as much in need of it as any.

The short service, at the end of which sweets but no presents were handed out, had followed a long procession through Chelsea, where the “ultimate Sloane area Christmas street party” was taking place.

We had trailed the saint through Peter Jones, a bookshop and another shop or two as he sought out his horse-drawn carriage, in which he would return to the church in splendour, drawing children behind him Pied Piper style, with children dragging reluctant parents away from the consumer heaven of Sloane Street and the King’s Road. “Look, a real St Nicholas,” remarked several surprised grown-ups as we passed.

Our St Nicholas was about as real as they get, and as jolly. The previous weekend he had led a similar celebration through Canterbury in his medieval bishop’s garb and the Archbishop’s son Pip sat beside him as his little helper. This Advent he has also visited Newcastle Cathedral, Maidstone Allington, St Matthew’s Westminster, Great St Mary’s Cambridge and even St Luke’s Church, Germantown, Philadelphia.

So who is the large, saintly but solitary figure heading this campaign to bring the legend of St Nicholas back to life and replace the consumerist impostor who has usurped him in the imaginations of millions of children worldwide?

Without wishing to detract from the magic of the occasion, I can reveal that the man beneath the mitre is none other than Canon Jim Rosenthal, a lay canon of the US Episcopal Church whose day job is as communications director for the Anglican Communion, currently riven by schisms of the kind the original 4th-century St Nicholas would have been all too familiar with as he witnessed his contemporaries being tortured and murdered by the Emperor Diocletian.

Rosenthal is co-author of St Nicholas: A Closer Look at Christmas. This generously illustrated history of the legend attempts to sort fact from myth and discusses what to do when a child loses faith in Father Christmas.

“The true St Nicholas, Bishop of Myra, Turkey, and saint in Bari, is becoming a more regular visitor to the UK,” says Rosenthal. “With more than 400 churches named for the saint in the Church of England the name is known but not necessarily the tradition.”

The Advent festivals are the work of the St Nicholas Society, founded by Rosenthal in 2000. The society is a sister organisation to the Sint Nicolaas Genootschap, based at Sint Niklaas near Antwerp and with another branch in The Netherlands. “It is the aim of the society to increase interest, learning and appreciation of the tradition of St Nicholas, patron saint of children, pawnbrokers and sailors, as the gift-giver, and the true and only Santa Claus or Father Christmas,” says Rosenthal.

“The society is not opposed to celebrations, gift-giving or other holiday activities, but it does encourage a sanity in the amount of gifts bought and exchanged as well as allowing Christmas to remain a day of Christian celebration. There is no need for the secularised Santa Claus or Father Christmas when St Nicholas, who was born in 260AD and died on December 6, 343, provides a perfect model for care and gift giving. Santa is a commercial invention. St Nicholas is a servant of Christ and an example for us all to enjoy and emulate.

“The society believes that St Nicholas helps young and old see what the true spirit of Advent and Christmas can be for us all, especially for those who find the holidays very stressful. It is our hope that jolly old St Nicholas will become, once again, in English-speaking parts of the world, a focus of celebration in his true identity. Santa is not bad, but St Nicholas is just better. I believe there is a bit of the spirit of St Nicholas in all of us.”

At Holy Trinity, after another day dealing with anathemas against the liberal West from evangelical archbishops in the southern hemisphere, Rosenthal says: “It is like changing persona, and becoming something extraordinary, so lovable, so appealing, except to children who immediately scream when they see me.”

Certainly my son Arthur, his little helper at Holy Trinity, did look at him with awe.

“Older people really love it,” Rosenthal continues. “Many on the street tell the kids: ‘He is the real one’. That is my goal, having kids know Santa and Father Christmas was real and had a name, Nicholas. He is not the focus of Christmas like Santa tends to be, but leads people to the Christmas Holy Child of Bethlehem.”

St Nicholas: A Closer Look at Christmas by Joe Wheeler and Jim Rosenthal is available from SPCK (020 7799 5083; http://spckonline.com) at £20.99