When you mention the Koran or the Holy Bible in a sentence, do you underline it, just like any other book title? For instance if you wrote, "The Koran and the Holy Bible have longed influenced people around the world" would you underline the Koran and Holy Bible? I have always learned that you don't underline them because they were not written by a specific person and/or they are sacred. Or, maybe you don't underline them because everyone knows what they are? I got in a debate with my teacher over it, and I really want to know that right answer.
Maybe you shouldn't underline the titles because the Koran and the Holy Bible have influenced people as symbols, and not simply as books. You wouldn't underline "the crecent" or "the crucifix."
I wouldn't underline the names of any books in that way. Maybe in italics, but I have rarely if ever seen the titles of books underlined in the middle of a sentence (although you sometimes get it in bibliographies). As for the Koran or the bible, I have certainly never seen them underlined.
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Underline or italicize titles of books, plays, films, long poems, magazines, CD-ROMs and any work usually published by itself. The main exceptions are the titles of sacred books (e.g., the books of the Bible, the Koran), titles of series (e.g., the Loeb Classical Library), and political documents (e.g., The Treaty of Versailles).
do u think we should underline any holy books, such as the Guru Granth Sahib, for the Sikhs
Interesting question. The Bible and the Koran are usually not underlined or put into italics. I suppose the rule would hold, philosophically, for the central texts of other religions. On the other hand, it may make understanding difficult for those who are not familiar with the title.

Six of one and half a dozen of the other: I have no definitive solution for you.
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But you are referring to The Bible as a text and not as a symbol. I'm guessing that you should underline it, as now we are more aware of it as text than a law. Would we underline Webster Dictionary though?