[1] While his work started in the domain of liberal Christian theology, its main thrust was always apologetical, seeking to defend religion against naturalist critiques.

[3] He attended the Gymnasium Andreanum in Hildesheim and studied at the universities of Erlangen and Göttingen, where he wrote his dissertation on Martin Luther's understanding of the Holy Spirit (Die Anschauung von heiligen Geiste bei Luther: Eine historisch-dogmatische Untersuchung), and his habilitation on Kant (Naturalistische und religiöse Weltansicht).

Otto's fascination with non-Christian religions was awakened during an extended trip from 1911 to 1912 through North Africa, Palestine, British India, China, Japan, and the United States.

In his early years Otto was most influenced by the German idealist theologian and philosopher Friedrich Schleiermacher and his conceptualization of the category of the religious as a type of emotion or consciousness irreducible to ethical or rational epistemologies.

[2] Otto's first book, Naturalism and Religion (1904) divides the world ontologically into the mental and the physical, a position reflecting Cartesian dualism.

[5] Otto's most famous work, The Idea of the Holy was one of the most successful German theological books of the 20th century, has never gone out of print, and is available in about 20 languages.

This mental state "presents itself as ganz Andere,[6][7][8] wholly other, a condition absolutely sui generis and incomparable whereby the human being finds himself utterly abashed.

[17] Otto's views can be seen[clarification needed] in the noted Catholic theologian Karl Rahner's presentation of man as a being of transcendence.

[citation needed] More recently, Otto has also influenced the American Franciscan friar and inspirational speaker Richard Rohr.

[20] The Iranian-American Sufi religious studies scholar and public intellectual Reza Aslan understands religion as "an institutionalized system of symbols and metaphors [...] with which a community of faith can share with each other their numinous encounter with the Divine Presence.

[citation needed] The eminent Romanian-American historian of religion and philosopher Mircea Eliade used the concepts from The Idea of the Holy as the starting point for his own 1954 book, The Sacred and the Profane.

[12][23] The paradigm represented by Otto and Eliade was then heavily criticized for viewing religion as a sui generis category,[12] until around 1990, when it began to see a resurgence as a result of its phenomenological aspects becoming more apparent.

"[27] Other philosophers influenced by Otto included Martin Heidegger,[17] Leo Strauss,[17] Hans-Georg Gadamer (who was critical when younger but respectful in his old age),[citation needed] Max Scheler,[17] Edmund Husserl,[17] Joachim Wach,[3][17] and Hans Jonas.

[citation needed] The war veteran and writer Ernst Jünger and the historian and scientist Joseph Needham also cited Otto's influence.