Perhaps it could be said that all of humanity is seemingly subject to degrees of mental illness, because, simply put, mental illness at its core is the perception of a self that lives outside of its own Source. Any part of the Son of God that identifies as only a body vs Spirit or Self with a capital S is self-chained by the small will to the belief in the idea of death. All belief is temporary and subject to dissolution – no beliefs can last. Even so, death of belief is feared while the belief in death prevails, and thus the mind is seemingly trapped by its own upside-down/backwards thinking.
Because the body can cease to be, it can never be a source of lasting happiness and thus it can never truly be a source of any kind of happiness (happiness that doesn’t last is not real happiness); and yet, by way of listening to ego, it is heartily believed to be so. Only mind believing itself to be ego, separated from eternal peace, could perceive of something that decays and dies to be a source of happiness. Death of Spirit or Self is not possible, and so the belief in the body always dies with that Knowledge.
Mental health on Earth, or as it is defined in ACIM- “corrected perception”- is the loss of the fear of death by ever increasing degrees of Self-Realization. Self-Realization illuminates the temporary nature of the body and the eternal nature of the Higher Self, and with this knowledge goes happily the belief in the body as a source of happiness. The mind, rejoined with it’s Creator in its rightful place with no belief in separation from Peace, is not at home believing itself to be a helpless ego in a body, but as its own Self.