Religion is a complex phenomenon. Despite its many manifestations and its ecstatic, emotional, and spiritual rewards, it also poses formidable challenges for human life on earth. As a result, the study of religion is inherently dialectical: scholars are constantly reworking and extending their notions of the concept, while its participants are constantly working to enlarge the scope of what is considered religious. The dialogue between these two aspects of the study is what makes the discipline unique. In the course of this process, a variety of univocal definitions of religion have been proposed. Attempts to formulate such a definition can be misleading. As the definition is applied to different historical materials, the results are invariably open to criticism and revision.

A definition that tries to describe the nature of religion by listing the various characteristics that it supposedly encompasses can quickly turn into a kind of lowest common denominator, a classification that fails to capture the true diversity of human experience. Such a definition is inevitably unhelpful for a discipline that relies on comparative history.

There is no single definition that will be adequate for all times and places, and it would be absurd to suggest that there is. A more helpful approach is to look for a way to organize the many diverse religions into a social genus and then seek to understand the particular characteristics that each of these has in order to discover their explanatory power. This is the polythetic approach to religion, which was pioneered by Ninian Smart and others.

While a number of social scientists have developed and refined polythetic approaches, most of the work on religion has been done by people who have sought to understand religion in terms of its beliefs. This has been criticized as a sort of Protestant bias, since beliefs are only one facet of the phenomenon. In addition, some argue that to analyze religion in terms of beliefs ignores the fact that all religions have institutions and disciplinary practices that shape beliefs.

The result of the search for a polythetic notion of religion has been an emphasis on the importance of studying religion as practice. It has been suggested that this view of the concept of religion helps to explain its enduring power and its ability to engender a deep sense of morality amongst its followers. Emile Durkheim, for example, defines religion as whatever group of practices unite people into a moral community, whether or not these involve belief in unusual realities.

Regardless of the precise definition that is ultimately chosen, there is no doubt that religion is a vital part of the human condition. It enables individuals to recognize the many kinds of limitation that may be imposed on their project of life and provides them with the means to transcend them (whether sacrificially, ecstatically, prayerfully, puritanically, or ritually). As such it is an essential element in the human project of self-transformation and transcendence. For these reasons, it is important to understand the ways in which religions function in history as well as to consider the problems they create and confront.