A fixed belief is any belief that someone holds regardless of whether there is evidence for or against it. While a fixed belief can be true, those tend not to be noticed because they are almost indistinguishable from evidence-based beliefs – although if the evidence changes, a true fixed belief may well become a false one.
Someone who holds fixed beliefs may actively defend those beliefs against rational criticism – a practice Issuepedia calls counterfactualism.
Fixed beliefs seem to arise most commonly because of membership in a belief-clique, i.e. adopting beliefs because of what others around you believe rather than because of evidence supporting those beliefs.
Tendency towards error
The overwhelming majority of fixed beliefs that come to light are false. This is almost statistically inevitable, since there are infinitely more false statements than true ones, and it takes constant updating of one's beliefs to keep them consistent with accumulating evidence and knowledge. Failure to update one's beliefs, even if they are initially consistent with available knowledge, makes it much more likely that some of them will become false at some point.
For example, if someone had fixed beliefs in creationism, geocentrism, or the idea that continental drift is pseudoscience – each of which represented the scientific consensus at some point in time – that person would have eventually had three profoundly false beliefs about the nature of the universe.
People who hold fixed beliefs often engage in proselytizing, for purposes of both cliquian signalling and self-reinforcement of beliefs which may be difficult to maintain (cognitive dissonance) in the face of available evidence and the disagreement or disapproval of non-believers.