Why do people think Christians are so weak and stupid?

There are many brilliant people I know that are Christians/believe in God; so why do atheists and agnostics personally attack someones intellectual capacity when they believe something different? Is there no other argument than to say someone needs a "crutch" or lacks the mental ability to reason correctly? I am a Christian and do not consider myself a stupid person. I am able to evaluate opinions and facts; why is your opinion without REAL tangible, scientific proof any more valid than my opinion without REAL, tangible, scientific proof?