Its origins and evolution
The tradition traces back to way before America was ever populated by Christians of any sort. In fact, it's estimated that people have been putting up Christmas trees for nearly 600 years. The first documented use of any sort of indoor tree happened in Estonia in 1441. An association of foreign merchants and other bachelor tradesmen known as the Brotherhood of Blackheads erected a spruce tree in their brotherhood house in the country then known as Livonia. On Christmas Eve, they'd walk the tree over to the Town Hall Square and danced around it. By the end of the 16th century, these young men had established a tradition of drinking, singing, and dancing around the tree, and finally, at the end of the evening, setting fire to it. Sounds like Christmas used to be one heck of a party back in the day.
Germany took on a few holiday indoor trees during the middle ages, although they were used exclusively in churches to represent the tree of knowledge in mystery plays. In the 16th century, guild halls would put up trees and let children decorate them. Wealthy Protestant families later adopted the tradition of setting up trees in their own homes, and by the 18th century they were decorating them with real burning candles--a much more dangerous counterpart to the little colored LEDs we use now. Candles were an expensive luxury item, so lighting up a whole tree with them was as much about showing off how much money you had as it was about getting into the Christmas spirit. It was sort of the 18th century equivalent of those people who coat the outside of their houses with as many electric lights as humanly possible.
The Christmas tree spread out of Germany into the rest of Europe during the 19th century, and even made its way over to Canada by way of Brunswick soldiers who held a smashingly popular Christmas party. Various accounts state that German settlers first set up a tree in the United States in the early 1800s, either in Pennsylvania or Boston. The tradition gradually evolved into the one we know and love today, with all its blinking lights and colorful ornaments. But were it not for the Germans, we would never have had something we now think of as so American.