Otto's problem in Das Heilige is that the word holy has been colonised by ethics. In ordinary use it now means morally perfect, and Otto argues that this is a late refinement covering over an older and stranger content: an encounter with something wholly other, before which the appropriate response is not admiration but a kind of terror mixed with helpless attraction.
Since no available word carried that meaning cleanly, he made one. From the Latin numen, a divine power, he formed the numinous, and he described the experience of it with a Latin formula that has been quoted ever since: mysterium tremendum et fascinans. The mysterium is what cannot be grasped or conceived; the tremendum is the awefulness, the overpoweringness, the sense of one's own nothingness that he called creature feeling; the fascinans is the pull, the strange desire to approach the very thing that appals. He gathered examples from the Hebrew prophets, the book of Job, Luther, the Bhagavad Gita and reports of ordinary people in ordinary places.
The influence has been enormous and the criticism is equally serious. If the holy is a category of its own, irreducible to anything else, then it is insulated by definition from psychological, social or historical explanation, and a critic can reasonably ask whether that is a discovery or a fence. The atlas records both the term, which is genuinely useful, and the objection, which has never been answered to everyone's satisfaction.