Here is the quote from myself:
...religion provides an undefinable mental, or rather spiritual state to the individual human person. The first scholarly attempt to describe this experience is found in the writings of Rudolph Otto.
Rudolf Otto (1869-1937), published a book called Das Heilige, or in English The Idea of the Holy, in 1917. Otto was one of the first scholars to study comparative religions in order to understand what all religions had in common. Despite the age of this book, it is still read and studied by anyone who is serious about the study of religion. Rudolf Otto not only read about religions, he travelled extensively around Europe, North America and in the Far and Middle East in order to experience other religions first hand. The first English language translation of his most famous book, The Idea of the Holy, was published in 1923. A second, and better translation was published in 1950, which is available through the Trellis library system. You can find online copies at the Internet Archive site.
Otto proposed that the essence of religion lies in this concept, which is non-rational (n.b. this does not mean irrational), and does not include ethics either. That is, he was concerned to define the word holy apart from any idea that holy=good. He did not deny that holiness could cause people to act in a good manner, to act ethically, but that ethical behaviour was an adjunct to the holy, and not of its essence. This essence was also non-rational, that is its essence does not lie in rational cause and effect relationships, but in presence - it was just present.
For this mental state Otto coined the term numinous. The numinous state - the holy without added ethical considerations and indefinable by means of reason was, to use an old but necessary term sui generis - meaning it is in a category by itself. It is unique, in the pure sense of this word. Today people speak of things or events or people as being very unique or more unique. Properly speaking, however, the word unique means one of a kind - a person, place, thing, experience is either unique or it is not. The numinous is unique.
What is it then?
The numinous is a feeling, an emotion-like experience, but not one of the standard emotions. Otto uses a Latin phrase he invented to label this emotion-like experience: mysterium tremendum. Mysterium means mystery, or that which the human mind cannot know or understand. Tremendum means fearful, or terrible. He uses Latin words because this fear is not a normal fear, rather it is a sensation more related to awe, than fright. Tremendum is a Latin gerund used as an adjective, but it comes from the Latin word for earthquake or trembling - so the phrase coined by Otto tries to describe a kind of internal, spiritual earthquake.
The numinous is a sensation similar to the feeling produced by music, or poetry, or perhaps in a horror movie though in this last as often a pleasurable feeling more than one of dread.
Not every person who is religious experiences the mysterium tremendum and even for those who do, it is not usually frequent. But the numinous is perhaps the essential component of religiosity, beyond doctrine, structure, prayer or any of the usual defined characteristics.
For a detailed summary of Rudolf Otto's ideas read this Short Essay by John Durham.