"Santa" did get many characteristics from Coca Cola and Dicken's a christmas Carol - like rosy cheeks and jolly laugh. He didn't come from St. Nicholas, probably, but he did borrow many traits from older religions.
"Santa" was even a common name for Nimrod (the first prominant sun-god) in Asia Minor according to Langer's Encyclopedia of World history.
Besides figgy pudding and the Dickens-additions, most Christmas traditions (trees, yule logs, candy canes, wreathes, gifts, the madonna and child with halos around heads, etc) have orgins with either Babylon or other cultures who took up after, most notably among them Egypt and the druids.
"Because the saint's life is so unreliably documented, Pope Paul VI ordered the feast of Saint Nicholas dropped from the official Roman Catholic calendar in 1969." ("Santa Claus" Microsoft Encarta Encyclopedia 99)
Nicholas' existence is not attested by any historical document, so nothing certain is known of his life except that he was probably bishop of Myra in the fourth century. . .
("Nicholas, Saint" Encyclopaedia Britannica 99)
"Thor was the god of the peasants and the common people. He was represented as an elderly man, jovial and friendly, of heavy build, with a long white beard. His element was the fire, his color red. The rumble and roar of thunder were said to be caused by the rolling of his chariot, for he alone among the gods never rode on horseback but drove in a chariot drawn by two white goats (called Cracker and Gnasher). He was fighting the giants of ice and snow, and thus became the Yule-god. He was said to live in the "Northland" where he had his palace among icebergs. By our pagan forefathers he was considered as the cheerful and friendly god, never harming the humans but rather helping and protecting them. The fireplace in every home was especially sacred to him, and he was said to come down through the chimney into his element, the fire." (Guerber, H.A. Myths of Northern Lands. New York: American Book Company, 1895, p. 61)
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. ("Santa Claus" Encyclopaedia Britannica 99)
As for the Dec 25th date, that stems back to the time of Nimrod, and every iteration of sun-worship holidays afterwards as the language/people were divided afterwards. (Horus, Tammuz, Molech, Mithra, Saturn, Kronos, Baal, etc). It was chosen by the Catholic church in an attempt to convert the pagans. The best Biblical support (and recent astronomical finds, very cool) point to late September. Christ actually could NOT have been born in winter, as there were sheep out in the fields.
There are many reasons Christians should not embrace Christmas: if the two biggest holidays the Christian community celebrates are known to just about everyone (except christians it seems) as pagan knock-offs - the Bible loses all credibility. No one is going to listen to a lecture on textual critisicm or archeology, personal testimonies, the evidence from history or eyewitness accounts, from geology or astronomy, etc - if all it looks to be on the surface is yet another iteration of the sun-god festival that cares more about presents and tree decoration than truth.
By analogy: if I were going to celebrate someone's birthday, and didn't know when it was - I wouldn't arbitrarily pick the day of a guy they hated and who tried to overthrow them and who had children burnt alive. Nor would I celebrate it with the other guy's favorite colors and decorations and presents, and then invite my friend to the celebration so he could watch everyone pretend the whole shebang was about him while we ignored what he had asked us to be about doing to begin with.
"The day was not one of the early feasts of the Christian church. In fact the observance of birthdays was condemned as a heathen custom repugnant to Christians," The American Book of Days, by George W. Douglas.
As to Santa being Satan: Nimrod/Gilgamesh attempted to overthrow God in anger because of the flood, and it was probably Nimrod who commisioned the tower of Babel (in apocryphal writings, in attempt to escape any future worldwide floods). And "It was Thor who in the last days of heathenism was regarded as the chief antagonist of Christ." Scandinavian Mythology
In certain German children’s games, the Saint Nicholas figure itself is the Dark Helper, a devil who wants to punish children, but is stopped from doing so by Christ. (Renterghem, Tony van. When Santa Was a Shaman. St. Paul: Llewellyn Publications, 1995, p. 105)
"The Two Babylons" is always a good place to start (albeit, take his anti-catholic bias with a grain of salt, but the scholarly work is pretty good).
Langer’s Encyclopedia of World History "Santa"
Davidson, H.R. Ellis. Scandinavian Mythology. New York: Peter Bedrick Books, 1982, p. 133