A note on the religious aspect, I think the deliberate use of holiday instead of Christmas is a corporate/political way to stay neutral. I personally don't use it not because I don't want to but because I never really thought about it. It would be interesting to see how a player would approach in the game and in their storytelling.
Hypothetically, if I had to assume a sim had a religion, I would think it would be in accordance to their personality and what's vital to them; i.e. my sims love of gummy bear pancakes. Being that he's a playful sim, he has to frequently eat those pancakes, his office is a shrine to playfulness, there's statues, mood lighted, music etc. All these things that a vital to his well being. His wife is similar there's inspiration decoration all over the house because she's creative, its vital for her to feel inspired on a regular.
In away we all say that our sims are not religious because there's no domination or name for it or there is no in-game god to pray to. But if you really pay attention sims are very religious because there are rituals they need to do daily to avoid being miserable and hiding under the covers all day.
