Belief is not alway irrational. A large number of biologists believe in evolution. A large number of geologists believe that the earth is x billion years old. A large percentage of the Western world believes that the earth orbits the sun. Are those beliefs irrational, simply because someone believes them? Is something instantly irrational *just because someone believes it*?
Evolution can be backed up with evidence. That the earth is X billion years old can be backed up by evidence. That the earth orbits the sun can be backed up by evidence. Creationism can't be backed up by evidence, it is un-verifiable and un-falsifiable. Belief itself is not irrational. Belief that cannot be backed up can be.
-- gkhan