I think it was shoehorned into Christianity to keep the pagan tradition alive ...
Maybe a tad post-modern in outlook?
And I think generally, although the pagan icons were employed, they were incorporated into Christian practice at a folk-level. As an old monk once said to me, 'people need their symbols'. So I think the process was organic, the product of two worlds rubbing shoulders, as it were.
The giving of presents and stockings hung from the fireplace goes back to the legends of St Nicholas, 4th century. The prototype of Santa Claus/Father Christmas.
The birth of Christ is a time of celebration, but the Mass is a solemn occasion, so one can see that Christians simply incorporated upbeat pagan practices, decorating their houses, for example. The Yule Log is Nordic, trees Germanic, mistletoe is Druid, but these are all late European practices. Candles are Hebrew as well as pagan.
The legend of St Nicholas was the blueprint for Sinterklaas in the Netherlands. Sinterklaas was portrayed as a trim figure, white robe, red cape, bishop's mitre, white hair and beard. He rode a white horse. The Dutch celebrated a tradition in his honour of putting coins in shoes left outside the homes of the poor. By the 16th century, St Nicholas/Sinterklaas is popular across Europe. His feast is December 6, the memorial of the martyrdom of Nicholas, bishop of Myra (year is uncertain, but the date is sure).
The Reformation saw the Protestants ban all Catholic 'superstition', but their drab and dismal celebration finds little favour. The Reformers can't get rid of the practices, so they rebrand them. He is to represent the generic spirit of good cheer, but to be associated with local folklore rather than Catholic legend.
In the 19th century St Nicholas crosses the Atlantic. Washington Irving, in
A History of New York describes a 'portly, bearded man who smokes a pipe'. It was Irving who made Santa slide down the chimney too.
John Pintard, founder of the New York Historical Society, commissions artist Alexander Anderson to sketch the saint. His is a trim figure in long robes and halo. The illustration is accompanied by familiar tales, a smiling child next to a crying and pouting one; a fireplace with stockings hung on either side. An accompanying poem describes Saint Nicholas as a giver of gifts.
Clement Clarke Moore, professor at the General Theological Seminary of the Protestant Episcopal Church, writes a poem for his daughters
An Account of a Visit from St Nicholas which becomes popular as
The Night before Christmas. His St Nicholas has a merry laugh, a twinkle in the eye, and a sleigh drawn by reindeer.
Thomas Nast, editorial cartoonist and recognised as 'the Father of the American Cartoon' illustrates Santa Claus in scenes including toys, a workshop at the North Pole. Also lists of children naughty and nice.
Sugar Plums, an American confectionary brand, creates an advertisement that shows an elf-like Santa in a red jacket, red-and-green hat and white bloomers, riding a little green sleigh being pulled by vari-coloured reindeer.
Nast's last version features 'Merry Old Santa Claus' as a round-cheeked, cheerful man with a long white beard, gold stopwatch, bag full of toys, slender pipe and a red coat. He is wearing a crown of mistletoe.
Twentieth century: Norman Rockwell creates many Santas, sad, tired, confused, but it's the jolly ones that stick.
Rockwell submits sketches of Santa to Coca Cola for use in their Christmas ads, but they get their in-house artist, Haddon Sundblom, to create their version. This is the Santa we know today, clothed in Coke's colours. The modern legend is that this Santa is a barely-disguised Sundblom self-portrait. He first appears 1931, but is repeated every year in The New Yorker, Saturday Evening Post and National Geographic, year on year from '31 on. This is the power of big-budget corporate branding.
The imagery of a 'white Christmas' so fixed in the minds of Brits and Bing Crosby fans is almost entirely down to Charles Dickens, who was writing in the time of the Little Ice Age when every winter saw snows in London, the Thames freezing over, etc.