Where were the doctrines of Purgatory and Penance prior to Gregory I.
Why did Gregory deny He was a Pope?
I believe, not 100% sure, that was in the book of maccabees, one of those 7 books that were thrown out by Protestants that still remain in the Catholic Bible.
Purgatory is not specifically mentioned, but it does decribe a “place” for atonement which came to ne known as Purgatory.
I believe Purgatory derives from the word “purge”