Can a Saint-to-be experience mental illness? The answer is yes and St. John of God, whose feast day is celebrated today, was one.
The first clue is that he led a “wild life” until he was about 40. Then, after realizing he had lived a life of sin, decided to repent of his wrong-doings by freeing all the Christian slaves in Muslim North Africa, and there be martyred. Today we might interpret such behaviors as possibly being manifestations of mania (though some might suggest simply a wayward man and overzealous penitent). Fortunately, John was wise enough to seek out a confessor. That confessor convinced him to open a bookstore instead.
In the early 1500s, when John lived, mental health counseling did not exist. Neither did anti-psychotic or mood stabilizing medicines. While now we know spiritual advice alone does not cure mental illness, at that time there was nothing else. So, whatever penance his confessor gave the future Saint John it did not assuage his irrational guilt. One day, after hearing John of Avila preach he felt moved to show sorrow for his sins by running through the streets, tearing his hair out, beating himself, and otherwise acting bizarrely. Consequently, he was put into a mental institution which amounted to being chained, flogged, and starved. Fortunately, John of Avila heard about this, visited, and persuaded John to use his energy caring for the sick and poor.
John of God is said to have had several visions during his life. Whether these were psychotic manifestations of his mental disorder will forever be unknown. But even if they were, God clearly used them to create good and spur St. John of God to sanctity. God can bring forth good from all things. No exceptions!
Picture by Francisco Camilo, 1653. Public domain, on Wikimedia. Many pictures of St. John of God include a crown of thorns, which I find appropriate when talking about mental illness.