A lot of what we believe is wrong. Communally shared beliefs are often wrong. Why do I think this? Here is one line of argument:
Many have a belief in deities, which has been called by some “The God delusion.” Now, there are multiple conceptions of what the deity or deities may be, call the number of them “N.” There are various incompatible views, and logically they can not all be correct. It does not follow that any are correct. So, we might uncharitably label most (N-1?) of these shared beliefs as collective delusions. This leads to the essential question: “How do you distinguish between a delusion and a simple false belief?” Is there any real and objective test? What are the implications for some types of mental illness?