How do you distinguish between religious fervor and mental illness?

My colleagues and I often care for patients suffering from hallucinations, prophesying, and claiming to speak with God, among other symptoms—in mental health care, it’s sometimes very difficult to tell apart religious belief from mental illness.

Part of this is because the classification of mental illness often relies on subjective criteria. We can’t diagnose many mental health conditions with brain scans or blood tests. Our conclusions frequently stem from the behaviors we see before us.

Take an example of a man who walks into an emergency department, mumbling incoherently. He says he’s hearing voices in his head, but insists there’s nothing wrong with him. He hasn’t used any drugs or alcohol. If he were to be evaluated by mental health professionals, there’s a good chance he might be diagnosed with a psychotic disorder like schizophrenia.

But what if that same man were deeply religious? What if his incomprehensible language was speaking in tongues? If he could hear Jesus speaking to him? He might also insist nothing were wrong with him. After all, he’s practicing his faith.