Religious beliefs are a complex and pervasive part of human society. Despite their diversity, many religions share some similar characteristics that are useful in understanding them. These characteristics include the nature of gods and spirits, myths, dogmas, and rituals. They also include the belief that all the things of the world are sacred or divine and the way they interact with each other and with the profane, or mundane, realm.
As the concept of religion grew and shifted, scholars began to study it as an academic discipline. This shift raised two important philosophical issues about how the category of religion should be understood: Should it be analyzed as an evolving social taxon with a fixed essence, or as a family-resemblance concept?
Substantive definitions of religion (as exemplified by Paul Tillich and Emile Durkheim) are those that determine membership in the class of religion on the basis of belief in some distinctive kind of reality. Increasingly, however, scholars have been adopting polythetic definitions of religion. These recognize many properties that are common to religions, but not essential to them.
While the latter approach has its advantages, it raises a number of problems for both methodological and theoretical reasons. It is often criticized for not being sufficiently rigorous and for overstating the degree to which certain practices are related to religion. It also can imply that there is only one prototypical religion, which in turn creates the problem of how to explain why some religions have certain characteristics and not others.