How devoted are you to your beliefs?

People often become rather devoted to their beliefs and will defend them at all costs. When one holds a belief, it is not a bad thing to mount a defence of it when attacked, as it can help to show you just how well you understand what it is you believe. But there are also the faith warriors who will mount a defence with any seemingly absurd line of reasoning they can muster. They will conjure up what they call scientific proof that doesn't even begin to pass muster with the scientific method, and they will engage in extreme logical fallacy while accusing others of doing the same thing.

Just how devoted to our beliefs should we be and how much interest should we have in questioning and defending our beliefs, to our selves, or others?

Do you think you and/or others should have some basis for what they believe, or is it pretty much anything goes, we can believe whatever we want no matter what science or facts may indicate?

Why is it, and/or how is it, that some people can become so devoted to beliefs that to the rest of us seem so patently ridiculous against the "facts" that we generally seem to agree on? Answers with a psychological bent get extra Karma points on this last one.