@edy4g Hmmm maybe so. Sieg definitely seemed pretty Cristian, what with calling Christmas "the birth of the Messiah, the son of god". Ritz is definitely not, but usually, when visiting relatives who follow a different religion you would go with the flow and attend service together(I've done so on numerous occasions). It seems likely there is no Christian church in RItz's hometown, so Sieg hasn't been for a while, but I'd imagine she would end up going with her family as per tradition.
That being said, we only saw the dinner party, so they might have gone in the morning. I just thought it was interesting that there was no mention of actual piety in this Christmas chapter, even though the author so far seems to want to make the manga feel "historically accurate" in regards to how people live day-to-day. From what I understand, religiosity would be the most important aspect of Christmas in the past, though the way it's covered here seems to focus more on a modern viewpoint (more a family event than a religious one, lots of Christmas-themed gimmicks). That being said, I don't have any context from the past, either, and this is an alternate universe. Nor am I Christian.