Part of the allure of religion is that it suppresses one's perception of one's own ignorance.
It's a scary world out there, and it was even scarier back when religions arose. Back then it was virtually impossible to acquire knowledge of anything, without a lot of work, and/or access to information, all of which was not readily available, much less capable of being obtained.
So having a book handed to you with the explanation that it contains everything you need to know is very comforting, allowing you to believe that you are far more in control of your scary world than you would be otherwise. It's delusional, but for most people not being hit by lightning, or succumbing to infectious diseases, or being killed by natural disasters, there is no reason to believe otherwise. And those exceptions, well, they can be rationalized away too, simply by referring to this book.
With such a handy dandy guide to life, you really don't need to even think about or acknowledge your real ignorance.
Life might still be a bitch, but at least you feel good about yourself.
See also: "The Dunning-Kruger Effect."