Religion is a human institution for expressing and transmitting values, beliefs, and attitudes that are central to the lives of its adherents. Many people regard their religion as the most important influence in their lives and are willing to live according to its teachings and, at times, die for them. Others see it as a source of comfort and guidance. Regardless of how they are viewed, there is little doubt that religions have made a significant contribution to the world’s culture and to humanity’s knowledge of the cosmos.

Religions are extremely diverse. They may be large-scale and coherently organized, with a clear hierarchy of popes, cardinals, bishops, priests, and laity; or they may be loosely structured. They may be based on sacred objects, such as a holy book or object of worship; on sacred actions and rituals, including prayer, sacrament, and devotion; or on a spiritual hierarchy of apostles, saints, martyrs, and gurus. Some religions believe in the existence of a supreme god or gods; others do not.

What unites them all, however, is their focus on a transcendent reality or the idea of salvation. Religions provide moral codes for the way people should behave, often in a hierarchical system of church and state, with leaders who gain almost godlike status. They also encourage social control through strict rules and regulations, for example, on what is allowed in a marriage or what is permissible sexual behavior.

Theories about the origin of religion are numerous. One popular view is that it arose out of human curiosity about the ultimate mysteries of life and death, and from a fear of forces that cannot be controlled. In this view, religion is like a safety net that gives believers hope in difficult times by promising eternal life and divine protection. Psychologists and neuroscientists (people who study the brain and nervous system) have argued that there is a psychological need for religion and that it fosters positive illusions, which give meaning and direction to people’s lives.

The word religion derives from the Latin religio, which can mean “to bind,” as in binding people to a set of beliefs and practices. The problem with trying to find a definition of religion is that, if it is too broad, it will cover only the various forms of Christianity, Hinduism, Judaism, and other religious traditions, which have different beliefs, attitudes, and practices, but the same basic core elements. Similarly, if the definition is too narrow, it will exclude religions that may well have valuable aspects.

In an effort to find an adequate notion of religion, some scholars have developed a functional definition that drops the concept of belief in a distinctive kind of reality and looks instead at what role religion plays in human life. For example, Emile Durkheim defined religion as whatever system of practices unites a group into a single moral community, and Paul Tillich used a similar approach to define the term. These are called “monothetic” definitions because they operate under the classical theory that a concept will have just one property that distinguishes it from other concepts.