Can somebody please explain why nouns like "the Devil", "the Pope", "the Holy Spirit" need the word "the"?

Aren’t they all proper nouns and therefore supposed to not need the definite article?

The devil’s name isn’t “Devil.” it’s Satan or Lucifer or Beelzebub, etc. The Pope’s name isn’t “Pope.” It’s Leo XIV or Robert Francis Prevos.

I’m pretty sure they’re all distinct entities. There’s a whole Pantheon of demons, devils, and so forth. Lucifer took a third of the Angels with Her.

Preface: I’m not a christian. And, of the religion teachers I’ve had, at least one had certain *ehum* interesting interpretations about the books, so my understanding of the lore may contain a bit of non-canon fanfiction. (I’m not from a very religious area, but she was a bit weird)

If I’ve understood correctly, “the devil” is more of a title, but not exactly? At least, “devil” should only refer to a single character at a time, and it should be obvious which one.
But, there are many demons. Still, if it’s common to refer to a certain demon, or if it’s apparent from the context, “the demon” could be fine, even if “demon” is very generic. If I tell my friends “You can’t believe what the idiot did this time.”, they know what I’m talking about, even though there are lots of idiots out there.

with Her

Is “her” the correct pronoun? A lot of art depicts Lucifer as pretty masculine, so perhaps “he”? Though, I’d probably use the neutral “they”.
I’m not sure if this is from the official canon, but one religion teacher taught that angels were genderless, and the “weird” one taught that they were both male and female, possessing the parts from both sexes at the same time.

Angels have been depicted in many ways, but I think that I like the weird teacher’s futanari version the most.