Dude, it's just a "what if?" story inspired by Japanese urban legends and Shintoism, which is a different Religion than Christianity.
You seem to take great offense at something Japanese taking cues from... Japanese culture?
I agree it has a nihilistic tone, it was never one of my favorites, it's not high on my list of 2000s anime I'm real nostalgic for (but I'm glad for anything from the decade that is still well remembered by younger weebs), but a lot of manga has a nihilistic tone and sometimes that can be fun, but I prefer stuff like Gantz which has loads of nudity and gore, Death Note feels a bit tame when compared to something like that, it's "babby's first edgy manga", it's an edgy manga on training wheels.
But you can enjoy a piece of media without agreeing with it's worldview or morals, you're like a Woke person in getting so upset that something doesn't share your views and decrying fans as "bad people" for liking something, get over yourself, pearl clutching over media is irritating behavior whether it's coming from the right or the left.
Also, a work of art can explore themes without necessarily fully endorsing it, just because a work explores nihilism doesn't mean the creator's intent is to say "yes, this is the only correct way of looking at the world", I mean look at the world around you today and tell me nihilism isn't a tempting worldview, why shouldn't art deal with it?