Rudolf Otto was a German philosopher, famous for his exploration of religious experience in his book, The Idea of the Holy (1917).
Otto was a Christian theologian and had written about the rational side of religion (such as the famous arguments for the existence of God), but in this book he writes about the irrational side of religion - religious experience, which he regards as the experience of the numinous (divine power)..
Otto's word numinous looks a bit like Kant's phrase Noumenon/noumenal reality - but don't be fooled, they're completely different things.
non-rational, non-sensory experience or feeling whose primary and immediate object is outside the self - Rudolf Otto
mysterium tremendum et fascinans (fearful and fascinating mystery)
this fear is due to the in-compatibility between man's egotism and the divine purity, between man's self-aggravated separateness and the infinity of God - Aldous Huxley
Otto's concept is explained by C.S. Lewis with the example of a tiger.
Suppose you were told there was a tiger in the next room: you would know that you were in danger and would probably feel fear. But if you were told "There is a ghost in the next room," and believed it, you would feel, indeed, what is often called fear, but of a different kind.
With a tiger you feel ordinary fear, because a tiger might kill you. People are frightened of ghosts too, but for different reasons. Ghosts are not dangerous - instead they are "uncanny" (strange, weird, unnatural, upsetting, unimaginable) and instead of fear, we feel "dread" in the presence of a ghost.
no one is primarily afraid of what a ghost may do to him, but of the mere fact that it is a ghost. It is "uncanny" rather than dangerous, and the special kind of fear it excites may be called Dread. With the Uncanny one has reached the fringes of the Numinous.
You might have seen some horror films where ghosts are in fact dangerous. But real ghost stories aren't like the movies.
If ghosts get across the idea that it's possible to be terrified of something that isn't physically dangerous, you're halfway towards the idea of the numinous. The word "dread" describes this nearly-numinous state - fear of something that's uncanny and might be dangerous.
Now suppose that you were told simply "There is a mighty spirit in the room," and believed it. Your feelings would then be even less like the mere fear of danger: but the disturbance would be profound. You would feel wonder and a certain shrinking... This feeling may be described as awe, and the object which excites it as the Numinous - C.S. Lewis
Lewis uses "wonder" and "shrinking" instead of fascination and terror, but he links these emotions to the term "awe".
Awe is another word that gets overused these days - "awesome" is used by some people to mean "quite good". But when Otto links religious experience to a feeling of awe, he's talking about an overwhelming urge to drop to your knees and hide your face.
Then suddenly the Mole felt a great Awe fall upon him, an awe that turned his muscles to water, bowed his head, and rooted his feet to the ground. It was no panic terror— indeed he felt wonderfully at peace and happy— but it was an awe that smote and held him and, without seeing, he knew it could only mean that some august Presence was very, very near.
- 'Rat!' [Mole] found breath to whisper, shaking. 'Are you afraid?'
- 'Afraid?' murmured the Rat, his eyes shining with unutterable love. 'Afraid! Of Him? O, never, never! And yet— and yet— O, Mole, I am afraid!'
The fear of the Lord is the beginning of Wisdom - Proverbs 9:10
Remove the sandals from your feet, for the place where you stand is holy ground - Exodus 3: 5
Holy, holy, holy, Lord God of power and might, heaven and earth are full of your glory
For you alone are the Holy One, you alone are the Lord, you alone are the Most High
Otto's ideas inspired lots of other thinkers, from psychologists like Carl Jung to writers like Aldous Huxley and rock bands like Pink Floyd. His idea of mysterium tremendum fascinans seems to sum up the nature of religious experience in the Bible and the Abrahamic religions (Judaism, Christianity, Islam).
Even if Otto's ideas to describe the theistic religious experience quite well, they don't describe the monist tradition so well. Religious experience in Buddhism isn't connected to terror or fascination, but to calmness and inner peace. Even in theism, the mystical tradition is of one-ness and unity, not the "wholly other".
The psychological experience of "awe" seems to be linked to the brain's temporal lobe - it features strongly in the hallucinations brought on by TLE and it is similar to the sensation created by Persinger's 'God Helmet'. This suggests it can be interpreted naturalistically and is of no religious significance.