A fiction. If you're assuming there really is nothing, it won't make sense. Religion isn't about accepting what you think is makebelieve. It's about putting faith in something you can't necessarily see or touch. The religion here gives you something more tangible.

Science can't explain everything, not yet. There are a lot of cases of people who should have died surviving what should have killed them, for example. We assume there is a scientific explanation behind it. Maybe there is. Maybe it's something bigger than science. You can't see their side if you're labeling their beliefs as fiction from the start.