Shhhh! Don't tell the School Thought Police, but a Christmas Tree is religious, too. It's based on a custom called the Jesse Tree ("And there shall come forth a rod out of the root of Jesse, and a flower shall rise up out of his root . . ."). You hang symbols of the lineage of Christ on an evergreen tree during Advent. A Jesse Tree is depicted on the stained-glass windows of Chartres Cathedral, from the 13th century.
Merry Christmas!
Thank you for that. However the practice of bringing evergreen trees and boughs indoors and decorating them around the time of the Winter Solstice goes back much farther than that. The Romans did this to celebrate Saturnalia and many pagan European cultures were doing something similar before Christianity came to Europe.