Belief in God is perfectly rational. For a thorough treatment, see Alvin Plantinga's "Warrant" trilogy, in particular "Warranted Christian Belief" available online in its entirety at the preceding link. It's more than a bit technical and it's not exactly light reading, but it rescues theistic belief from the charge of irrationality (or, at least, from certain charges of irrationality - certain factual charges are outside the book's scope).