Rudolf Otto was a German theologian and philosopher, best known for his work on the concept of the numinous, which refers to a profound spiritual experience that evokes feelings of awe and wonder. His influential book, "The Idea of the Holy," published in 1917, explores the nature of religious experience and the feelings associated with the divine. Otto's ideas have significantly impacted the fields of theology, philosophy, and religious studies. He argued that the experience of the numinous is a fundamental aspect of all religions, shaping how people perceive and relate to the divine. His work continues to be studied and discussed in contemporary discussions of spirituality.