Nordic pagans and Celtic Druids revered the evergreen tree as a symbol of everlasting life and hope for the return of spring. While other plants and trees died, the evergreen trees remained alive continually hence, they were revered as manifestations of deity.

As a symbol of prosperity, the Druids decorated the evergreen outdoors. The Scandinavian pagans were the first to bring the decorated trees indoors and the Saxons (a Germanic pagan tribe) were the first to use candles to illuminate the trees.

In addition to these pagan rituals, the tree is also linked to the celebration of the Winter Solstice. Pagans would celebrate the Winter Solstice and as a part of that celebration, they would decorate trees.

Christians adopted the tradition as they did many other Pagan traditions. Call it what you want, its a tree.