Q. Isn't it okay to use "delusional" simply to mean "stubbornly rejecting strong evidence"?
A. No, it's not.
Here are the *first two* paragraphs of the Wikipedia entry on delusion:
A delusion is a false belief held with absolute conviction despite superior evidence.[1] Unlike hallucinations, delusions are always pathological (the result of anillness or illness process).[1] As a pathology, it is distinct from a belief based on false or incomplete information, dogma, poor memory, illusion, or other effects ofperception.
Delusions typically occur in the context of neurological or mental illness, although they are not tied to any particular disease and have been found to occur in the context of many pathological states (both physical and mental). However, they are of particular diagnostic importance in psychotic disorders includingschizophrenia, paraphrenia, manic episodes of bipolar disorder, and psychotic depression.
Labeling a group "delusional" is hate-filled and can lead to USSR-like conditions.
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Okay, there are two very concise, core answers on my part.
Your turn!
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Q. What would you say to somebody proposing that theism be classed as a psychiatric disorder?