Rudolf Otto
Rudolf Otto explored the idea that the experience of sacred power is the fundamental fact of human religious experience. When we experience sacred/holy power (what Otto calls the numinous) we feel gripped by a reality that is "wholly other;" that is we are in some way overwhelmed by the feeling that we are encountering something that is radically and fundamentally different from us and from the ordinary world.  Consequently, encountering the sacred evokes a powerful, non-rational and emotional response. In other words, the sacred is a mysterious something that both frightens and fascinates at the same time.  It is a mysterious sense of dread, awe, majesty and wrath as well as love, pity, mercy, joy, peace and a blessedness. Otto used the Latin phrase mysterium tremendum to summarize the main components of the experience of sacred power:
  • It is an experience of the "wholly other"--entirely different from anything we experience in ordinary life. It evokes a reaction of silence.
  • It is an experience of fear and terror. It provokes terror because the holy presents itself as overwhelming power
  • Finally, the numinous presents itself as merciful and gracious.

 
According to Otto's interpretation, the sense of dread and awe forms the foundation of a religious response in the form of worship.  In particular, one who has experienced the sacred responds with  acts of expiation (an act of sacrifice to remove pollution or sin) and propitiation (sacrifice to appease the deity).
A classic example of Otto's idea of the sacred may be found in Isaiah 6 in the Hebrew Scriptures where the experience of Isaiah's vision  in the temple in Jerusalem is recorded:

In the year that King Uzziah died, I saw the Lord, sitting on a throne, high and lifted up. And the hem of his garment filled the temple. Above it stood the seraphim: each one had six wings: with two t hey covered their faces, with two t hey covered their feet and with two t hey flew.  And one cried unto another and said:


Isaiah by Michaelangelo

Holy, holy holy is the LORD of hosts:
 the whole earth is full of his glory.

And the posts of the door shook at the voice of those who called and the house filled with smoke. Then I said, "Woe is me! for I am lost; because I am a man of unclean lips, and I dwell in the midst of a people of unclean lips: yet my eyes have seen the King, the LORD of hosts."

Then one of the seraphs flew to me, holding a live coal that had been taken from the altar with a pair of tongs. He touched my mouth with it and said, "Now that this has touched your lops, your guilt has departed and your sin is blotted out."

Notice in this event that the prophet experiences something "wholly different" that cannot be fully described. The distinctness of the reality he experiences is emphasized by the repetition of the world "holy" which in Hebrew means "set apart."  In addition, his reaction is one of terror as he is overwhelmed by the power of the sacred. Finally, however, his experience is one of grace as he receives mercy and purification.

Otto believed that over time, humanity attempted to rationalize and systematize the experience of  mysterium tremendum. Consequently doctrines (authoritative teachings) emerged to explain an irrational experience in more rational terms. For example, the doctrine of the atonement in Christianity can be viewed as a "rational" reflection of the powerful encounter with the sacred that leads to a sense of unworthiness and thus to a need for expiation.