Well sounds like you agree with me to an extent, so much for my small mindedness and ignorance. How do YOU determine which cases of mental illness are due to sin and what cases are not?
I don't. I won't blame the sufferer for their mental illness any more than I would blame someone who had cancer, MS, or Parkinson's disease. I don't blame the people who are struck by lighting or die in an auto accident either.
Are all healthy people blameless? Then don't try to pretend that everyone who is ill is under the wrath of God. Job was a man who worshipped God so faithfully that God bragged about him. He lost everything he had and was reduced to sitting in ashes scraping his sores with a broken piece of pottery. Was God punishing him? No. God tested him.
You stated that "mental illness is only caused by sin" which is where I strongly object. Every sin seperates us from God. We are all under indictment and all deserve punishment for our sins. To claim that all people who suffer in a specific way is a result of their specific sins is arrogance.