But there are certain ideas that will be downvoted and dismissed because people feel like they aren't useful to be talking about, like if God exists.

The problem with "does God exist" isn't about the fact that LW is atheist. It's that it's hard to say interesting things about the subject and provide a well argued case.

I don't expect to learn something new when I read another post about whether or not God exists. If someone knows the subject well enough to tell me something new, then there no problem with them writing a post to communicate that insight.