IO9 had an article about the way prior assumptions and beliefs affect our perception. If you are prone to believe that the world is filled with miracles, you may believe you have experienced them. But conversely, if you are persuaded in advance that there is no such thing as the supernatural, that too can distort your perception – in ways that can be scientifically studied!
In theory, many people – whether Christians, atheists, or something else – try to remain open to changing their mind in light of new evidence. In practice, it doesn’t always work out that way. And sometimes people will advocate or attack underlying assumptions precisely because that can either bring the entire house of cards crumbling down, or keep one from looking at it too closely because of confidence in the foundation upon which it is constructed.
I’ll close by linking to a post of Jerry Coyne’s from a while back. It included the chart below, which puts Darwin’s theory of evolution (in terms of percentage of people who “believe in” it) right between ghosts and the virgin birth. Make of that what you will.