Theories of demonic possession and the belief that exorcism is an effective cure have done unimaginable damage to countless people suffering from mental illness over the centuries. I think it would be fair to say that these approaches have led to many of the unhealthy attitudes many people still have toward those struggling with mental illness. This is an important and very sad lesson about the evils to which religion has led (and continues to lead).
Just when we think that modern science has finally displaced these dangerous myths, we learn that there are religious believers in positions of power who are determined to return us all to the Dark Ages in the name of their preferred religion. Thus, we should not be surprised that even now there are Roman Catholic bishops insisting that we need more exorcists and even more of this superstitious bullshit! And sadly, the belief in demons is not limited to Catholics.