the Encyclopedia Britannica, the legend of Santa Claus began with Saint Nicholas, a saint from the fourth century.

Traditionally speaking, Saint Nicholas was born in the ancient Lycian seaport city of Patara, and, when young, he traveled to Palestine and Egypt. He became bishop of Myra soon after returning to Lycia. He was imprisoned during the Roman emperor Diocletian's persecution of Christians but was released under the rule of Emperor Constantine the Great and attended the first Council (325) of Nicaea.

After his death he was buried in his church at Myra, and by the sixth century his shrine there had become well known. In 1087, Italian sailors or merchants stole his alleged remains from Myra and took them to Bari, Italy; this removal greatly increased the saint's popularity in Europe, and Bari became one of the most crowded of all pilgrimage centers. Nicholas' relics remain enshrined in the 11th-century basilica of San Nicola, Bari.

Saint Nicholas became well known for his generosity and kindness and legends of miracles he performed for the poor and unhappy grew. News of his good deeds abound. It is said he gave generous marriage dowries of gold to three girls whom poverty would otherwise have been extremely poor. He restored to life three children who had been chopped up by a butcher and put in a brine tub.
In the Middle Ages, devotion to Nicholas extended to all parts of Europe. He became the patron saint of Russia and Greece; of charitable fraternities and guilds; of children, sailors, unmarried girls, merchants, and pawnbrokers; and of such cities as Fribourg, Switz., and Moscow.

Thousands of European churches were dedicated to him, one as early as the sixth century, built by the Roman emperor Justinian I, at Constantinople (now Istanbul). Nicholas' miracles were a favorite subject for medieval artists and liturgical plays, and his traditional feast day was the occasion for the ceremonies of the Boy Bishop, a widespread European custom in which a boy was elected bishop and reigned until Holy Innocents' Day (December 28).

After the Reformation, Nicholas' cult disappeared in all the Protestant countries of Europe except Holland, where his legend persisted as Sinterklaas (a Dutch variant of the name Saint Nicholas). Dutch colonists took this tradition with them to New Amsterdam (now New York City) in the American colonies in the 17th century.
Sinterklaas was adopted by the country's English-speaking majority under the name Santa Claus, and his legend of a kindly old man was united with old Nordic folktales of a magician who punished naughty children and rewarded good children with presents.
Today, the magic of Santa Claus is not limited to any religion. The power of Santa’s legend affects children, parents, and grandparents from all over the world. “Santa”, today, is an international and inter-denominational spirit of giving and helping the needy and deserving, and a symbol of hope and generosity. In that light, he is truly magical and immortal.