The word 'delusion' is interesting. It is often used to describe anyone who disagrees with you. 'You are totally delusional'. This debate has been completely hijacked by US politics by Don and his cronies as if he is a post modern hero. All nations are delusional- they see the world from where they stand and feel that is their God given right and objective truth.
In Salvador in Brazil, they often put plates of food at crossroads to appease the gods, they believe. To say it is all unscientific is undoubtedly true but to say the act of preparing and placing it there happens is also true, it is observable scientific fact. The belief that most people there have in the power of religion and spirits affects the behaviour of the society there, from the architecture of its buildings to when people have the cigarette breaks, it affects people's habitual behaviour. So it has power, not because of magic, but because it shapes the place and its life. To be there one has to accept the reality the delusion creates. It only takes many to believe in it for it to have real effects.
It is not necessary to believe in a delusion to see the reality of its effects.
I use Salvador as an example, but it could be any community shaped by its dogma - from a supposedly upright churchgoing community in rural England to Riyadh.