There are various legends about how the tree represents the enduring message of Christ, but actually, as with many of the accepted and generally unquestioned traditions of Christianity, this too was lifted from the already existing pagan customs and conveniently incorporated as an integral religious symbol.
It was customary for the pagans in the northern hemisphere to celebrate the winter solstice. This is the shortest day and longest night of the year, falling on 21 or 22 December, and the pagans believed it to be the turning point when the Sun god began to recover from the sickness that had plagued him in earlier months and which brought winter to the land.
This was one reason for so honoring them, and, secondly, their greenery symbolized the other plants/crops/means of livelihood that would, thanks to the recovering Sun God, soon spring back to life, and make overall existence easier for everyone.
This was the general philosophy that was followed by the Celts, the Goths, the Vikings, the early Romans and the Egyptians, amongst others of the ancient world. The Egyptians, worshipers of the Sun god Ra, of course used the more easily available green palm leaves instead of pines.
The Romans had a really grand celebration known as the Saturnalia, where the entire city turned out to pay homage to their God of Agriculture, Saturn, and have a generally riotous time. All Roman houses and temples were decorated with evergreen boughs during Saturnalia. Later on, the Christians adopted the custom.
But the tradition of a decorated Christmas tree, started in the 16th century by the Protestant reformer priest, Martin Luther. Apparently, on one of his night walks, he had stopped to admire the sight of stars shining brightly through the branches of the towering evergreens and thought his family should share in the wonder of the sight.
So, instead of logically summoning them outside, he later had a tree cut and brought indoors, where he attempted to recreate the effect by fixing candles in candle-holders on its branches. This was the start of having indoor Christmas trees.
Much later, when many of the Protestant followers of Martin Luther migrated to Pennsylvania, they brought the tradition of the Christmas tree to America.
It didn't gain wide-spread hold in America right away though. The Puritans, suspicious as always and rightly in this case, wanted to have nothing to do with with 'heathen customs', which included a strong opposition to singing carols and anything that might remotely look like enjoyment and 'desecrate' the holy event.
The Puritans were opposed to celebrating Christmas on 25th December too and even managed to get the General Court of Massachusetts pass a law that made it a penal offense such that people could actually be arrested for singing or decorating their homes!
However, as more and more new people from Europe and the rest of the world came to settle in America, bringing with them their own notions of how Christmas ought to be celebrated, the Puritans gradually loosened up, and by the 1840s, Christians were going to prisons for reasons that usually had little to do with religious fervor.
Meanwhile, in England, the Christmas tree received official sanction, again with German influence, when the German-born Prince Albert, the husband of the very popular Queen Victoria, arranged for a family Christmas tree in 1846. With the Royals setting the fashion trend thus, the rest of the country was quick to follow.
The Christmas Tree soon became a common sight at Christmas, with the wealthy outdoing each other in setting up lavishly decorated, floor-to-ceiling Christmas trees. It was only a short step from here to start setting up the trees outside in public.
Even these decorations have by now come to gain a symbolic meaning in the Christian religion, the apples and crystal balls representing the fruit of redemption, the lights signaling the triumph of good over evil, the bells ringing in the joys of life, and the star or dove on top epitomizing the holy spirit.