Why the bible?

Another question I've often posed is why the bible? Different religions spring up every time someone adds a book to a current religion (Muslim = Christian + Mohammed's book). So why does the bible continue to have such a devout following?

The bible most Christians use is translated and known to have been modified (Emperor Constantine did it before he adopted Christianity). It isn't the oldest book we have. It also isn't the most vague. So why do people believe it?
I guess they just want to. This is fine, just don't critisize others if they want to believe in The Odessey.

It seems a natural course of "spiritual" evolution that humans would go from polytheism to monotheism and end up at athism. Guess we still have a to wait a bit.