I know what you mean. It can be a daunting task but dont let it get you down. Just start working on the clergies of a handful of deities. I'd say pick some that are opposed and work on a little backstory as to why they are opposed and what goals teach faith has.
The goals of each faith can help you come up with the backstory of each as well. Also don't feel that once you start on opne faith that you have to finish it right away. If while writing about one, you have an idea for another, save the document and write out your idea for the other. Either that or make sure tohave some scrap paper handy.