The winter solstice was celebrated in Europe by many pagan cultures. As these cultures were agrarian, farming was extremely vital to their livelihood and subsistence. Hence why the winter solstice became an important aspect of their calender year. In Roman religion Saturn was the agricultural deity, so a festival, the Saturnalia, was held during the period December 17th-23rd. However, Saturn could not do his job with the help of the Sun, so the Dies Natalis Solis Invicti, the Day of the Birth of the Unconquerable Sun was celebrated immediately after the Saturnalia on the December 25th. This was celebrated as the days were now getting noticeably longer again after the winter solstice.
During the rise of Christianity, one of the many obstacles the Church faced was the established traditions which were ingrained into Roman society. To combat this, the Church appropriated many of the festivals and their traditions and instituted their own festival. So the Church established the 25th as Jesus' birthday* and this made it easier for people to convert to Christianity as they could still engage in the same traditions as they had done previously. However the bible strictly forbids this kind of appropriation.
Jeremiah 10: 1-5 (NIV)
"Do not learn the ways of the nations** or be terrified by signs in the heavens, though the nations are terrified by them. For the practices of the peoples are worthless; they cut a tree out of the forest, and a craftsman shapes it with his chisel. they adorn it with silver and gold; they fasten it with hammer and nails so it will not totter. Like a scarecrow in a cucumber field, their idols cannot speak they must be carried because they cannot walk. Do not fear them; they can do no harm nor can they do any good.”
So not only is concept of appropriation of previously established rituals forbidden, but the bible specifically uses the Christmas tree as an example. As an evergreen tree, it was originally used by the pagans as a symbol of eternal life, and survival through harsh winters. They brought in indoors and decorated it.
So every Christian around the world, not only celebrates Jesus' birthday at the wrong time of year but also, as a supposed tribute to their God, goes against his wishes and erects a tree and decorates it.
*Jesus is a quasi-sun god, for instance the halo can be found on Helios and Apollo, not to mention that halo comes from the Greek Halos meaning "disk of light around the sun or moon".
** I do not speak Hebrew but the Septuagint and Vulgate both use words which roughly translate as race or nation of people. Probably best translated as Gentiles, so it is clear that it is referring to all who are not Jewish. KJV translates it as heathens.
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