For me, "religion" generally means "belief in invisible people/spirits/beings that have influence on the world". Superstition is a superset of that that includes the subset of "belief in ghosts/spirits/souls aka, invisible people/spirits/beings that have influence on the world". The majority of Japanese (and probably most other cultures) believe in "ghosts/spirits/souls that have influence on the world" but most Japanese might not a few special all powerful ones (eg: "God").