The dictionary supplied with Microsoft Word defines a
delusion as ‘a persistent false belief held in the face of strong
contradictory evidence, especially as a symptom of psychiatric disorder’.
The first part captures religious faith perfectly. As to
whether it is a symptom of a psychiatric disorder, I am inclined to
follow Robert M. Pirsig, author of Zen and the Art of Motorcycle
Maintenance, when he said, ‘When one person suffers from a
delusion, it is called insanity. When many people suffer from
a delusion it is called Religion.’