No, simply believing in them is not justification for a diagnosis of mental illness. But what about those who hold conversations with them? Those who base their actions on a voice in their head? Are they sane? The Son of Sam serial killer didn't really want to kill anyone. He was just following the commands of his dog, wasn't he? How is he any less sane than someone who believes he is talking with a god and who acts on that belief? Should such a person not be held responsible because it happened to be God's voice he was following?