Well, something else has crossed my desk that demanded response. It was a meme that claimed the "cure" for demonic possession is atheism, as no atheists have ever become possessed. Believing in demons is the cause of possession, according to this bumper-sticker like picture. This claim brings up some interesting theological and philosophical issues worth discussing.
Now let's put aside the question of whether there HAVE been cases of supposed demonic possession in atheists (there have) and let me establish that though I do believe in something like demons, I do not believe in demonic possession as, say, a Roman Catholic exorcist would understand the term. I will assume here there is no such thing as posession and atheists can't even experience the phenomenon others label possession. Would that be evidence against the RC's proposition that possession actually happens? No, not in the slightest.
That is because demons are assumed to be persons, and persons have motivations, motivations that can be hypothetically explored in plausible ways. It would be very easy for an RC to explain away the "fact" that atheists do not get possessed. Demons would have no reason to possess them. For demonic possession is understood to be the way in which dark powers challenge our belief that God does or even can love us. And an atheist is already lacking such a belief.
This brings up the difficulty when trying to use something like scientific evidence to study ANY religious phenomenon. For God, too, is understood to be a person, with goals and plans and the ability to act on those plans. It is possible to concieve of good reasons WHY God may not want us to be able to absolutely prove His existence, and so it may be impossible to do so. Both Immanuel Kant and Dietrich Bonhoeffer came up with such reasons.
A person can hide. A person is not predictable in the way most natural phenomenon are. Psychology is often a mess of a discipline for just this reason. Drugs that are known to be ineffective become ingrained in the treatment structure of mental disorders because of increased placebo effects. Entire fields of study remain even when they produce no discernible results because, darn it, we have to DO something to help people, and people are hard to help.
A parallel can be found in studies on ESP. I don't believe in it. But the stories and examples that prompt study in it...a mother knows her child is dying many miles away, someone dreams of impending danger, Phillip K Dick's mysterious diagnosis of his child....are examples of extreme stress situations that cannot be reproduced in a lab, not really.
The point is that the scientific method may not be equipped to evaluate these claims, at all. Evidence matters in these cases, but it is weak anecdotal evidence, not the hard knowledge-bringer of scientific study. Evidence matters to theists, too, if it didn't they wouldn't spend so much time trying to defend against the problem of evil. The problem is taken as evidence against their beliefs. But because God is conceived of personally, it is doubtful that the concept can be decided SCIENTIFICALLY.
That doesn't mean atheists have no REASONS for unbelief (nor theists for belief, to my mind). But those reasons are more about interpreting facts than facts themselves. Does order (such as the order we find in nature) require an orderer? Does order imply mind? The answer to this question is intuitive, a matter of rational and internal reflection, and cannot be decided finally on the evidence itself (since evidence is just and example of what is beings studied). In the end the point all sides in these debates have to face is that science has limits. That is why we call it metaphysics...because it is beyond physics.