1) You can't question faith.
The religious texts are believed despite mountains of evidence against them- If they were anything other than 'Holy' they would be sitting next to Harry Potter and Twilight right now. 'Because God said so' or 'Because I have faith' does not prove anything, anymore than me believing in Dragons because I saw one on a Welsh Flag and my flatmate told me the DO exist. It is also completely illogical for a society which relies in science to have such an attitude, I know enough from my boring statistics work that you need a LOT of evidence just to say 'this does not disprove the hypothesis'. But if I say 'you don't get healed by magic water' I'm ganna go to hell.
2) It stands in the way of scientific discoveries and evidence.
Religion prevents real science from being developed and understood by a significant amount of society, such as Geneses being taught in schools as fact, and Darwin being given as much regard as a crazy homeless man shouting in the street.