Sometimes people make experiences which they see in the context of a belief system. They call these experiences religious experiences, spiritual experiences, sacred experiences, or mystical experiences. William James made the concept popular, in the 19th century. He did this to fight the  growing rationalism of Western society. 

In many religions, such experieces are called revelations. This also applies to the knowledge that comes from them. These religions say that a god or gods caused these revelations, and that they are not a result of a natural cause. They are considered real encounters with God or gods, or real contact with higher-order realities of which humans are not ordinarily aware.

Skeptics say that these religious experiences are a normal feature of the human brain. As such, they can be studied in the same way other features of the brain are studied. To better be able to study them, scholars have classified such experiences in different ways.

Definitions 
Psychologist and philosopher William James described four characteristics of mystical experience in The Varieties of Religious Experience. According to James, such an experience is:

 Temporary- the person will soon return to a "normal" state. The experience may feel like it was outside time or space. 
 Diffiult to describe
 Insightful - usually people think they have learned something valuable. Very often, they also think that this kind of knowledge is usually hidden from human understanding. (called noetic)
 Passive - usually people cannot control the experience, it just happens to them. 

 Ecstasy – The believer has a soul or spirit which can leave the body. In ecstasy, the focus  is on the soul leaving the body. It will then make experiences that are different (called transcendental). This type of religious experience is characteristic for the shaman.
 Enthusiasm - The believer is not able to understand God. A sacred power enters the body of the believer and possesses it. A person who is able to handle this situation is often called a medium. The god or power will use the medium to give a message to the world. According to Lewis, this is the same as ecstasy, which is only one form of possesion.
 Mystical experience – Mystical experiences are in many ways the opposite of numinous experiences. In the mystical experience, all 'otherness' disappear and the believer becomes one with the transcendent. The believer discovers that he or she is not distinct from the cosmos, the deity or the other reality, but one with it. Zaehner has identified two distinctively different mystical experiences: natural and religious mystical experiences (Charlesworth: 1988). Natural mystical experiences are, for example, experiences of the 'deeper self' or experiences of oneness with nature. Zaehner argues that the experiences typical of 'natural mysticism' are quite different from the experiences typical of religious mysticism (Charlesworth: 1988). Natural mystical experiences are not considered to be religious experiences because they are not linked to a particular tradition, but natural mystical experiences are spiritual experiences that can have a profound effect on the individual.
 Spiritual awakening – A spiritual awakening usually involves a realization or opening to a sacred dimension of reality and may or may not be a religious experience. Often a spiritual awakening has lasting effects upon one's life. It may refer to any of a wide range of experiences including being born again, near-death experiences, and mystical experiences such as liberation and enlightenment.

History

Origins 
The notion of "religious experience" can be traced back to William James. He used the term "religious experience" in his book, The Varieties of Religious Experience. It is considered to be the classic work in the field. At professional conferences, people often cite this book. James makes a difference between institutional religion and personal religion. Institutional religion refers to the religious group or organization, and plays an important part in a society's culture. Personal religion, in which the individual has mystical experience, can be experienced regardless of the culture.

The term has been used before James, though. In the 18th, 19th, and 20th centuries, several historical figures said that religion and its beliefs can be grounded in experience itself. Many of these views had a lot of influence. Kant said that moral experience justified religious beliefs.  John Wesley also thought that the religious experiences in the Methodist movement (paralleling the Romantic Movement) were a basis to religious commitment as a way of life.

Criticism 
The notion of "experience" has been criticised.

The term "Religious empiricism" is seen as highly problematic. Karl Barth saidf that the term was wrong altogether. Some influential modern scholars holding this liberal theological view are Charles Raven and the Oxford physicist/theologian Charles Coulson.

Robert Sharf points out that "experience" is a typical Western term, which has found its way into Asian religiosity via western influences. The notion of "experience" introduces a false idea of duality between "experiencer" and "experienced". In kensho there is no duality of observer and observed. "Pure experience" does not exist; all experience is mediated by intellectual and cognitive activity. The specific teachings and practices of a given tradition may even determine what "experience" someone has; this means that this "experience" is not the proof of the teaching, but a result of the teaching. A pure consciousness without concepts, reached by "cleansing the doors of perception", would be an overwhelming chaos of sensory input without coherence.

Notes

References

Other websites 

 Stanford Encyclopedia of Philosophy, Religious Experience
 "Self-transcendence enhanced by removal of portions of the parietal-occipital cortex" Article from the Institute for the Biocultural Study of Religion
 Peru: Hell and Back National Geographic explores the uses of Ayahuasca in Shamanic healing
 Is This Your Brain On God? (May 2009 week long NPR series)
Spirituality
Religious behavior and experience
Harv and Sfn no-target errors
Pages with unreviewed translations