Here's the open-and-shut case against theism: People often tell stories to make themselves feel better. Many of these stories tell of various invisible and undetectable entities. Theory 1 is that all such stories are fabrications; Theory 2 is that an arbitrary one is true and the rest are fabrications. Theory 2 contains more burdensome detail but doesn't predict the data better than Theory 1.

Although to theists this isn't a very convincing argument, it is a knock-down argument if you're a Bayesian wannabe with sane priors.