In The Philosophy of Religion Based on Kant and Fries (1909), Otto follows closely Jakob Friedrich Fries.

It is critical of naturalistic views which, through reducing and explaining everything through mathematical-mechanical laws, excludes the categories of purpose and mystery both essential to religion. Otto’s Naturalism and Religion (1907) contrasted the naturalistic and the religious ways of interpreting the world. Mysticism East and West (1926) is a comparison of Indian religions and Christianity, in particular the notions of the mystical as religious experience within their respective traditions. Otto authored several works throughout his life, some of which focused on religious experience and Indian religions. During his travels, he began formulating his ideas, learned Sanskrit, studied the Bhagavad Gita, and translated several religious texts into German. For work purposes on the history of religions, Otto traveled across locations in North Africa, China, Japan, United States, and Palestine. Some of Otto’s important ideas, one of which was his idea of the Holy, were influenced by a trip he took in the early twentieth century. He later held numerous professorships, first of systematic theology at Göttingen, then theology at the University of Breslau, and finally systematic theology at the University of Marburg. Otto studied philosophy and theology at the University of Erlangen and at the University of Gottingen. He is well-known for his idea of religious experience as being the apprehension of the “Holy” or the “numinous,” concepts he presented in his book The Idea of the Holy (1917).

German theologian and philosopher Rudolf Otto (1869-1937) was an influential theorist within the historical development of religious studies.

The Idea of the Holy by Rudolf Otto