Religion is a system of beliefs and practices that define what people regard as sacred or spiritual. It is found in every culture and provides a common framework for understanding the universe. It offers hope, moral guidance, and a means of making sense of life and death. It promotes psychological and physical well-being and may motivate social change. In addition, it presents a range of possible goals to aim for, and it transmits the means for attaining these goals. These goals may be proximate—a more virtuous, charitable, or successful way of living, for example—or they may be ultimate in nature, such as the rebirth of the individual into a higher state of existence.

Despite its central role in human history, there is much disagreement about what religion is. Philosophers have offered a wide variety of definitions, some based on beliefs and others based on functional characteristics. For example, the 19th century German philosopher Emil Durkheim developed a definition based on the function of creating solidarity among a group. In his view, a religious person is one who has some dominant concern that organizes his or her values and that, in the case of Christianity, also involves belief in unusual realities.

Other philosophers have focused on the experiential nature of religion. The American philosopher William James argued that any phenomenon that is felt to be solemn and gravely important is a religion, irrespective of whether it has a supernatural explanation (James 1902:39). For example, he might have pointed to a wedding or funeral as an expression of such a belief.

The basic argument of sociobiology is that religions are early and for millennia have been successful protective systems, tied to the potentialities of brain and body and to the necessity of survival. They provide people with the confidence and security they need to explore other areas of possibility, such as their own natures, societies, and the world around them. These explorations are known as somatic explorations, from the Greek word for body.

In addition, it has become clear that not all beliefs in disembodied spirits or cosmological orders constitute religion. For example, there are people in the past and present who do not believe in either of these.

Other philosophers have tried to develop more careful and nuanced definitions of religion. Some, like the British philosopher John Polkinghorne (1988), have emphasized that religion is not just belief in a transcendent power but also acts of worship and service to it. These acts may involve prayers, offerings, and ceremonies such as sacraments, baptisms, communions, and marriages. They may also involve emotional and psychological states, such as crying, laughing, screaming, trancelike conditions, or feelings of awe and reverence. They often involve the use of ritual objects and the recitation of texts that have special meaning. The goal of these acts is to achieve a connection with what the person believes to be a supreme power or, in some religions, with a loving divinity.