Where does the line between "eccentric" and "mentally ill" cross?

I was just curious about where you draw the line between eccentricity and illness. I’ve seen people suggest to mentally ill people not to take their meds because doctors don’t appreciate eccentricity and want to eradicate any behavior that isn’t lockstep with the norm.

I love and value eccentricity myself – but mental illness is real. There are people who live substandard lives because they can’t function enough on a daily basis because their mental illness gets in the way.

One thing I try to wrap my head around is the business of people who hallucinate voices in their heads. In most contexts, we’d say they were mentally ill. But in some religious sects, they are blessed with hearing the voice of God.

How do we decide what is mental illness and what is simply eccentricity?