You are making a lot of mistaken assumptions about spirituality and how the Japanese see spirituality which is why you are confused. It's a common error here. It starts by assuming only christianity is legitimate. Then it extends to calling abyone with different beliefs a satan worshipper. By the tine you start actually investigating your investigation is polluted by these ideas and you arrive at a wholly inaccurate conclusion.
If it makes you feel any better, they are just as baffled by the stuff you consider to be true as you are of them.
I'm not asking you to stop making dumb assumptions because I enjoy watching this movie scene unfolding before me and laughing.
Protip: the Japanese weren't educated by the subverted failure that is the western education system and might know a few things you don't.