German theologian and philosopher Rudolf Otto (1869-1937) was an influential theorist within the historical development of religious studies. 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).
Otto studied philosophy and theology at the University of Erlangen and at the University of Gottingen. 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. 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. For work purposes on the history of religions, Otto traveled across locations in North Africa, China, Japan, United States, and Palestine. During his travels, he began formulating his ideas, learned Sanskrit, studied the Bhagavad Gita, and translated several religious texts into German.
Otto authored several works throughout his life, some of which focused on religious experience and Indian religions. 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’s Naturalism and Religion (1907) contrasted the naturalistic and the religious ways of interpreting the world. 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. In The Philosophy of Religion Based on Kant and Fries (1909), Otto follows closely Jakob Friedrich Fries. It also shows Kantian influences in how Otto argued that human beings have immediate knowledge of the noumenal world, which shows itself in “feelings of truth.”
Otto promoted what other believers considered to be liberal views. What contributed to this reputation was his goal to incorporate moments of numinous experience into church liturgy, which brought him opposition from Neo-orthodox theologians. However, Otto’s work has (both past and present) influenced a number of theorists across several fields, including Jewish spirituality, philosophy, psychology, feminism, phenomenology of religion, religious studies, and theology. His notion of the Holy has influenced how some influential thinkers within the Christian tradition, including Paul Tillich (1886-1965), Karl Barth (1886-1968), and C.S. Lewis (1898-1963), have understood and attempted to articulate the notion of religious experience. Otto’s idea of the Holy also lent support to the development of the study of religion because it reinforced the idea that religion constitutes a non-reducible, original category in its own right which deserves to be studied (which explains why his idea has been popular within the phenomenology of religion).
The Holy as the Essence of Religion
Otto was one of several prominent theorists to seek after an essence (sine qua non) of religion. An essence of religion is an essential core, fundamental component to religion that without which something would not be religion. Immanuel Kant’s (1724-1804) ideas were most influential in the way later scholars approached the topic of religion’s essence. Kant identified religion’s essence as consisting of three fundamental human capacities: the rational, ethical, and the beautiful. Kant appeared most interested in the ethical category, attempted to marry religion with ethical and moral sensitivities, and ultimately viewed religion to be a “matter of feeling.” He left later scholars with three possibilities:  to develop his own tripartite formula further (strengthening and/or expanding it),  to seek after alternative possibilities within the tripartite capacities (some scholars suggested that rationality and/or aesthetics to be a more appropriate category), or  to seek an altogether different paradigm.
A scholar such as Otto worked with this Kantian paradigm. According to Otto, religion is supremely transcendental and there are significant limitations when attempting to understand it through ratio-centric methods. In fact, he claimed that irrational elements belonged to the heart of religion and that by applying too much rationality one will produce an inaccurate portrayal of it. This motivated him to present the idea the Holy (also referred to as the “numinous”) which he believed is unique to religion, as well as closely associated with goodness and distinguishable from rationality. What resulted was a strong attempt on Otto’s part to trace the essence of religion not to rational, sub-rational, or super-rational elements but directly to irrational elements, which produced what he called divination, or irrational religious intuition. Without this core component religion would not exist.
The Holy (Numinous) as Religious Experience
Otto referred to the numinous, which he claimed is an intangible and unseen yet compelling reality that inspired both fascination and dread within human beings, that is always present within religious experience and awareness. It also encompasses the irrational and non-rational core of religion that points to a reality outside of oneself. In order to flesh out his category, Otto posited the numinous to consist of two elements that were bound together, the tremendum mysterium and the fascinans. By tremendum mysterium he meant awe, majesty, and urgency. By fascinans he meant something wholly other and distinct from everything else but despite such distinctiveness still attracts and fascinates. According to his evolutionary concept of religious consciousness, Otto believed that the first human beings acknowledged the numinous, but only the fearful side of it (as represented in their fear of divine wrath). They soon, however, became aware of another side of the numinous, namely, “positive self-surrender to the numen.”
Christianity as the Superior Religion
Although a number of contemporary religious and irreligious people will frown at Otto referring to religion as constituting something non-rational or irrational, he was ultimately motivated to defend religion. During the nineteenth century, religion was coming under increased criticism from scientists and historians working within the natural sciences and biblical criticism. Otto can be seen as attempting to legitimatize religion by providing it with a foundational basis within subjective experience which points to something objectively real beyond the human being. In this respect, he argued that Christianity was superior to all other “sister” religions because of its development and progress of the sacrum (numinous). Otto believed that religions only have value in their ideas of the sacrum and it is only in terms of the sacrum that they can be measured. Religion cannot be measured in its contributions to culture, attitudes toward society, or in any of its external manifestations. Rather, the most perfect religion is in its sacrum and the degree to which it has advanced in intuition and divination. Otto believed that Christianity is the perfect religion in this sense.