"The Bible" is typically used to refer to the collection of holy books of the Christian faith. It's a proper noun and it gets capitalised.
However, "bible" can also be used as a common noun, in sentences like "TAOCP is a programmer's bible", meaning some kind of an extensive and comprehensive manual. In that case it is not capitalised.
What about when it is used in phrases that refer to the holy texts of various religions (or cults, or whatever), like "the Christian Bible", "the Hebrew Bible", "the Discordian Bible"? Should those be capitalised?