I wonder about this as well...like maybe there's some comfort in automatically "diagnosing" someone when they might see patterns or think in ways that challenge your priors..
EDIT: Imagine being powerful and wealthy and assured in your position in the Catholic Church and someone comes along and questions geocentricity and says you're wrong. It's a pretty easy leap to huffily say well, they are "mentally ill, crazy, delusional, paranoid"
all kinds of authoritarian regimes and movements certainly do this.
that said... you just have to have a conversation with someone experiencing psychosis. it's a totally altered state of consciousness, they are sensing and understanding the world in a radically different way. it's never just an otherwise normal person with a nonconformist belief.
EDIT: Imagine being powerful and wealthy and assured in your position in the Catholic Church and someone comes along and questions geocentricity and says you're wrong. It's a pretty easy leap to huffily say well, they are "mentally ill, crazy, delusional, paranoid"