The article sketches a theoretical model which explains how it is possible that fundamentalist beliefs can emerge as a result of an individual rational adaptation to the context of special living conditions. The model is based on the insight that most of our knowledge is acquired by trusting the testimony of some kind of authority. If a social group is characterized by a high degree of mistrust towards the outer society or other groups, then the members of this group will rely solely on the authorities of their own group for their acquisition of knowledge. In this way they can adopt a corpus of beliefs which may seem absurd from an external point of view. However, they may be locked in a "fundamentalist equilibrium" in which particularistic trust, common sense plausibility, epistemic seclusion, social isolation and fundamentalist beliefs are mutually reinforcing – and in which individuals who adopt the "fundamentalist truths" of their group do not behave more irrationally than individuals in an open society who accept the "enlightened" worldview of their culture.