This trope is about Japanese works that portray Christian nuns as being similar to Miko (shrine maidens, shamans, spirit mediums). Few Japanese people today are overtly religious, and even fewer are Christians (less than two percent of the population). So Christianity, particularly the Roman Catholic type, tends to get used as an exotic religion, especially if you can mix in mythology and folklore. Some writers are fannish enough to do research, especially if a story takes place in a Medieval European Fantasy. But most have only a basic understanding of the religion, and few writers have a realistic idea of what a nun's life is like.