Who was the demon possessing Regan in "The Exorcist"?

I finished reading the book, and after that I watched the movie. I really have no imagination, so help me out here? Who was the demon possessing the girl in this? It calls itself The Devil, then later in the book it says it’s “a devil”. The demon offers no information at all about who he is.

I was thinking Pazuzu, as it’s the limestone statue representing said demon found in Northern Iraq in the opening, and that name is mentioned once more in the book and movie, but it is never directly attributed to the demon possessing Regan. All the relations in the book don’t help clear stuff up either, such as the old priest having battled the very same demon before. He never actually calls him “Pazuzu”.
So far as I know Pazuzu isn’t even from Christian lore. Not that this changes anything…so who was the demon in the story?

Incidentally as a second question, in the beginning of this story a church gets desecrated, it is allusioned to as a“black mass” by the characters in the book. Now I guess I wasn’t paying attention but…they never go back to this in the story. We never figure out who took a shit in the church or who drew dicks all over the statues. It wasn’t Regan, wasn’t Dennings…so who messed up the church and why is this never spoken of again? Was it just an excuse to read almost four pages attributed to what goes on in a black mass? (don’t get me wrong, it was awesome)