The paper first proposes a new definition of religion which features a novel four-layered element and which does not involve any circularity ; thereby, it allows to clearly distinguish the phenomenon of religion from certain other worldviews, in particular from certain political ideologies. Relying on the findings, the paper develops two structural conceptual models which serve to describe religious and non-religious belief systems. Further, the definition and the conceptual models allow to establish a clear criterion to distinguish pivotal structural differences (...) between religious and non-religious belief systems. The criterion is based on the concept of two kinds of rationality: first-level and second-level rationalities. These will demonstrate to what degree religion can be a rational enterprise, and what role logic can play in it. The result is a clear-cut line in the structures of religious and certain consistent non-religious belief systems. (shrink)