The Holy Bible is the Word of God. It’s not one of Clymer’s bibles of motorcycle mechanics, collectively known as Clymer’s Manuals. Notice the proper nouns are capialized. We capitalize Bible because when referring to the book titled Holy Bible, Bible is a proper noun. Titles of books are proper nouns. There is no exception for The Holy Bible, even when you shorten the title to simply Bible. It’s still a proper noun.
What is the ‘rule’?
I left my bible on the table.
vs
I left my Bible on the table.