Christianity and Christmas have lasted for about 2009 years now (historical say Jesus was born around 6 A.D), so why should 700 years change anything?
Seriously, sound better and find a smarter excuse
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1) Jesus was born a few years before AD. 2) Christmas is a religious celebration. Sure, others celebrate it, but there's problems with this theory. 2.1) Is that the same universe we live in? No. 2.2) do Guardians have their own "God" floating there? Yes. 2.3) if they had an actual representative "God" there, even if prior to the Travellers arrival, why would they believe in another faith? They wouldn't.