Wikipedia goes into some depth on the topic, describing how Christianity fused older traditions into the holiday, in the spirit of goodwill and gift-giving. https://en.wikipedia.org/wiki/Christmas#Observance_and_tradi...

Every year, some people need to marginalize Christianity's contribution to Christmas despite ample evidence to the contrary. I attribute it to an ignorance of how religion digests culture and evolves, plus the usual resentment towards that religion in particular.