St. Bruno's depiction in art and his reputation for holiness of life have led to his patronage against demonic possession.
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St. Bruno was the founder of the Carthusian Order and was highly regarded for his holy and virtuous life. He is widely known as the patron of exorcists and possessed persons and is invoked against demonic possession.
It is difficult to pinpoint the exact reason why St. Bruno receives this designation, but there is some evidence that can help us sort it out.
First of all, a famous depiction of St. Bruno recalls a scene from his life where he witnesses the funeral of Raymond Diocres, a seemingly virtuous teacher of scripture. At the funeral, Bruno looks upon the resurrected body of Raymond, who comes back to life to say that his soul has been condemned to Hell for his secret sins. Frequently this painting features a demon ready to accept the soul of Raymond.
Secondly, there is a medieval painting of St. Bruno casting out a spirit from a possessed woman. This story behind this painting is difficult to ascertain, but it reveals a legendary tradition of associating St. Bruno with an exorcism.
Third, St. Bruno was highly regarded for his holiness of life and his spiritual writings contain much insight into the spiritual life. His holiness and asceticism made him into a strong spiritual warrior, ready to combat any spiritual attacks.
Whatever the circumstances might have been that earned him the patronage of exorcists and against demonic possession, St. Bruno continues to be a powerful intercessor in heaven and can be of assistance in cases where demonic forces are at work.