Between clear cut cases of delusion and ordinary beliefs lies an interesting no man's land. It's difficult (arguably impossible: 1, 2) to clearly define a delusion, so the border between ideas that are and are not of psychiatric concern is uneasily guarded. One strand of this pragmatic policing involves considerations about how widely shared a candidate belief is. If lots of people believe something - even if that something is manifestly false or at odds with other culturally mainstream ideas - it is less likely to be judged a delusion.
This issue comes up infrequently in regular mental health work. Most clinically significant ideas are personal and are causing people terror. An individual who believes they are being injected with AIDS every night is almost certainly not in possession of a shared belief. Such a belief is first personal, not belonging to a broader cultural web. But in forensic psychiatry, where people often endorse unusual beliefs that get them into trouble, determining whether a belief represents a delusion can sometimes be more complex. It also has significant ethical and legal implications. Reasonably widespread but relatively obscure belief systems can fall into the psychopathological no man's land.
One distinctively North American example is the Sovereign Citizen movement, brought to my attention by a forensic clinical supervisor who has evaluated some adherents for their competency to stand trial. This is a terrain in which Sovereign Citizens are assessed with some frequency, as their beliefs bring them into direct conflict with the US legal system. There is no single coherent belief system (check out this link to the Southern Poverty Law Center,and the papers linked below for more information), but adherents believe a diverse mix of things about their relationship to the state. Broadly - as implied by the name - Sovereign Citizens take themselves to be technically legally independent of the government. For at least one strand of believers this is because they think that they were put up as collateral for US government debt when the dollar came off the gold standard in the 1930s. By cashing themselves in against this in some way Sovereign Citizens seem to hold that they can opt out of the country's laws.
As a result these individuals are not very cooperative participants in legal interactions. They present police with fake government documents to evade basic traffic regulations. They commit violent crimes but refuse to enter pleas. Their lack of cooperation sometimes extends to levels of disruption that require their ejection from court. It may be that their anti-government beliefs elevate the risk that they will break the law. They are frequently violent, and are regarded as a domestic terrorist threat in the US. For the seriously interested, there is a Reddit thread devoted to collecting (and mocking) their antics.
But what do mental health professionals make of these individuals? They certainly have idiosyncratic and over-valued ideas. Their behavior is sometimes described as "bizarre." When they put forward their ideas they talk in a strange pseudo-legal language that sounds idiosyncratic and grandiose, resembling what Silvano Arieti called "talking on stilts." But the limited available literature suggests a wariness to include them under the umbrella of the mentally disordered. Sovereign Citizens share their beliefs with other people; they are typically able (if not always willing) to converse with professionals, and they don't necessarily meet other criteria for mental health problems. A good case series can be found in this article by a US psychiatrist, and their spread into Canada is evidenced in this article by two University of Toronto psychologists. The case of Sovereign Citizens provides a fascinating example of a distinctively American extreme belief system somewhere between the religious and the legal. It also speaks against the worry that the country's mental health care is nothing more than a way of regulating political and social deviance.