Wow... Stephen King is religious? Thats kinda... off. id have assumed hed be an atheist. Ah well, im not actually a fan anyways.
Anyways. Usually id agree that it doesnt matter if the author is an atheist or a theist as long as the book is entertaining. But it does effect things sometimes, such as when the book actually has religious themes.
Take the Mistborn trilogy, which i was reading recently as my fantasy hunger started up. The whole thing has this theme whereby religions are used to control and pacify the populace. one character does nothing but go through hundreds of religions and prove them wrong or unsuitable AND by the end of the first book one of the characters has died in public but set up a plan (using a shapeshifter no one knows about) to fake coming back to life and start up a religion where hes worshipped. Those are the main examples, but there are more critical ones.
So i finished the books, and thought to myself. "Wow. Im atheist and hate all religions, but this guy is really could stand to be a little more subtle about his God-Hatred." Then, just for fun i looked him up, and the guys a fucking Mormon!
I was shocked! For a while I thought id accidentally misinterpreted three books worth of obvious religion-hate. To this day i dont understand how any theist could write such an atheist book.