Religion is an ancient system of beliefs and practices, typically involving worship of a supreme being. It ideally serves many functions, including social cohesion and stability, reinforcement of ethical values and norms, psychological well-being, the integration of personality and a sense of purpose in life, and participation in a communal ritual or ceremony.

Religious experiences are often intense and emotional, and may involve crying, screaming, trancelike conditions, feelings of oneness with others, and transformation. They may also be non-transformative, but nonetheless deeply moving. They are characterized by symbols, myths and legends, and may include sacred texts and ceremonies such as prayer and rites of passage (e.g., baptism and wedding).

It is often difficult to define religion. Some philosophers argue that the attempt to create a definition of religion is a waste of time and energy because there is such a huge diversity of belief systems and practices around the world. This argument has been strengthened by the fact that a univocal or unambiguous definition would quickly lead to a minimal notion of religion, which could not be useful in the study of religion.

Other philosophers have argued that to talk about religion in terms of beliefs or mental states is to fall into the trap of Protestantism, and that instead we should focus on institutions and disciplinary practices. This view has been strengthened by the recognition that many people do not regard themselves as members of any particular religion. It has also been strengthened by the work of sociologists such as Emile Durkheim, who emphasised that religions serve important social functions even if they do not have any supernatural or transcendent elements.