Imagine if, before you could make a big decision in your daily life, you had to fact check it by running it through a several hundred year old book about the legends of the Tooth Fairy to make sure that your decision matched what the Tooth Fairy would have done in that situation. Although if you stuck to it long enough, you would make consistent (if not, by people outside of this Tooth-Fairy-ism, seemingly reasonable) decisions, and could even reliably guide others to Tooth-Fairy based solutions to their problems, those decisions would still be based on absolute lunacy, and though they may be the 'right' thing to do Tooth-Fairy wise, they would rarely be the wise thing to do.
This is one of the problems any major religiously driven group is going to have. Of course they can make correct decisions on their own, and of course they can just pick and choose from their Tooth-Fairy-encyclopedia until they can construct an argument supporting their decision, but still, many of their decisions are just not going to be ones that help them. They are going to choose not to ally with some groups who they likely should, and then they are going to ally with others who they likely shouldn't. But above all else, they're going to solve their problems by just killing any non-Tooth Fairy believers.