Most of these stories have one thing in common: they are presented as a way of "tainting" the celebration that is supposed to "disprove" Christianity in some small measure, or at least be disapproved of by Christians. This attitude is actually quite peculiarly American, and ironically "Christian," as it is, by and large, the same critique levelled against Christmas traditions by the Puritans and other critics of the excesses of the season. The Roman Catholic church, the only Christian church in Europe until Martin Luther's attempt to reform it, openly acknowledged the pagan roots of many of its practices and doctrines, going so far as to accept Socrates and other Greek philosophers as "pre-Christian." It was the Protestants, in "protest" against all things "Catholic," who despised such reasoning and thought anything "pagan" "tainted" their religious practice. So let's set that aside right now, and move on.
1) The "Druid" connection: This actually comes from the Roman Catholic church. Per Penne Restad (Christmas in America, New York, Oxford University Press 1995, pp. 57-58): "One legend credited St. Winfred, reputed to have felled a giant oak that had been used for Druidic worship, with dedicating the evergreen tree to Christmas. 'This little tree,' he is said to have told a crowd of converts who looked on, 'shall be your holy tree to-night...Let this be called the tree of the Christ-child; gather about it...in your own homes; there is will shelter no deeds of blood, but loving gifts and rites of kindness.'"
Which is, frankly, so highly developed a statement about the Christmas tree we now know (as we'll see in a minute) that it's already suspect as a retrojection onto the dim and unknown past of a well settled custom that lacks only the patina of "tradition" to make it wholly acceptable.
2) It was "pagan" to begin with. Well, again, according to Restad: "In pre-Christian times, Romans used evergreens, symbols of fertility and regeneration, to trime their houses at the Kalends at the end of January. Eventually Christians appropriated the use of evergreens for their Christmas celebrations. To remove the taint of paganism, they associated it with new beginnings and man's second chance with God. The tree became for pious folk a representation of Jesus as the Light of the World, Tree of Life, and second Adam born to right the sins of the first." That connects with the "Paradise Tree" of Germany, of which more in a minute.
3) Martin Luther did it. A lovely legend, but, again, absolutely no evidence for it whatsoever.
4) The Germans did it. Very likely, but not because they worshipped trees (that was the Druids, and they didn't kill what they worshipped in order to keep it indoors for the winter). In fact, the earliest records of any trees being used in Christian celebrations is from 16th century Germany, as Francis Weiser explains:
CHRISTMAS Eve is the feast day of our first parents, Adam and Eve. They are commemorated as saints in the calendars of the Eastern churches (Greeks, Syrians, Copts). Under the influence of this Oriental practice, their venera-tion spread also to the West and became very popular toward the end of the first millennium of the Christian era. The Latin church has never officially introduced their feast, though it did not prohibit their popular veneration. In many old churches of Europe their statues may still be seen among the images of the saints. Boys and girls who bore the names of Adam and Eve (quite popular in past centuries) celebrated their "Name Day" with great rejoicing. In Germany the custom began in the sixteenth century of putting up a "paradise tree" in the homes to honor the first parents. This was a fir tree laden with apples, and from it developed the modern Christmas tree.The Paradeisbaum apparently came out of the stage settings for the morality plays which were very popular in Europe (the famous passion play in Oberammergau is a relic of those plays) . It is a short step indeed from the "Tree of the Knowledge of Good and Evil" to associating a tree with a Christian celebration. And while Judaism certainly had a long history of avoiding such syncretism, except for certain Protestant groups, Christianity has never considered it as much of a problem.
And how did the Christmas tree spread over America? Stephen Nissenbaum (The Battle for Christmas, Vintage, New York, 1996, pp. 176-218) argues quite persuasively that it spread not by immigrants and those who saw one, but by literature. Indeed, the stories I know from Appalachian dwellers setting up their first Christmas trees in the early 20th century conform to this pattern: it was something the mountain people read about, not something they saw in their neighbor's house.
As Nissenbaum sees it, in fact, the Christmas tree first gained popularity in America because it was seen as a moral symbol of how good children are rewarded for unselfish behavior. It's Nissenbaum's careful cultural history that serves as a good antidote to legendary tales like those of St. Winfred and the evergreen. Traditions always develop from roots and causes almost wholly unconnected with what we consider important. Which makes the whole search for origin as a search for "truth," an interesting and cautionary enterprise, indeed.