The German interpreter of religion Louis Karl Rudolf Otto (1869-1937) found a thread of unity among all religions while resisting attempts to account for religion in non-religious terms such as the moral, the rational, and the aesthetic.
Born in Peine (Hanover), Germany, in 1869, Rudolf Otto was educated at Erlangen and Göttingen and taught at the Universities of Göttingen and Breslau before becoming professor of systematic theology at the University of Marburg in 1917. In that same year he published Das Heilige (translated as The Idea of the Holy), one of the most significant books in religion in the first half of the 20th century. Illness forced his early retirement in 1929, and he died in 1937 of arteriosclerosis and physical/psychological consequences of a serious fall.
His life and work spanned a tempestuous period in the religious and political history of Germany: World War I, the Treaty of Versailles, the Weimar Republic, the rise of the National Socialist movement, and the election of Adolf Hitler as chancellor of Germany. During that period he resisted two strong challenges to religion—evolutionary naturalism and dogmatic, exclusive Christianity. Through that resistance he identified what is "religious" about any religion while recognizing and respecting the peculiar features of specific religions.
Otto is best known for his book on the Holy, which has been translated into Swedish, Spanish, Italian, Japanese, Dutch, French, and English. A British prelate, R. W. Mathews, noted in 1938 the broad impression of the book and suggested that it had an even deeper influence in England and America than in Germany.
Otto always understood himself as a Christian. He grew up in a pious Christian family and in his final lecture before retirement he referred to himself as a "pietistic Lutheran." Yet his thought and his travels both manifested and stimulated his interest in other religions, especially Judaism, Islam, Buddhism, and Hinduism. Judaism provided him the scriptural text (Isaiah 6:3) and the theme of the book on holiness. On a trip to Morocco in 1911 he was moved by a Sabbath service in a synagogue: "I have heard the Sanctus Sanctus Sanctus of the Cardinals in St. Peters, the Swiat Swiat Swiat in the Cathedral of the Kremlin and the Holy Holy Holy of the Patriarch in Jerusalem. In whatever language they resound, these most exalted words that have ever come from human lips always grip one in the depths of the soul with a mighty shudder, exciting and calling into play the mystery of the other world latent therein."
In The Idea of the Holy, Otto brought together interests he had pursued earlier: the dominance of the spirit over the letter in a study of Luther (Die Anschauung vom Heiligen Geiste bei Luther, 1898), the claim for a source of religion beyond evolutionary naturalism Naturalistische und religiöse Weltansicht, 1904), and the rejection of enlightenment rationalism as determinative of religion in favor of "feeling" as more decisive for religious awareness than rational knowledge or faith (Kantisch-Friessche Religionsphilosophie und ihre Anwendung auf die Theologie (1909). In this book, Otto isolated the quality of the "religious" which distinguishes it from the moral, the rational, and the aesthetic. He found that quality in the numen or, coining a word, in the numinous. This quality, he claimed, is present in all religions, usually in connection with other distinct qualities such as the rational and the moral, but it is neither derived from nor reducible to these other qualities. Otto was probably writing to counter explicitly Kant's Religion within the Bounds of Reason Alone. Otto spoke of the numen as non-rational, implying thereby that the numen is, in essence, not "good" or "beautiful."
Having identified the primal quality of the religious as the numinous, Otto developed an understanding of the Holy as a complex category, characteristic of all high religions, containing moral, rational, and aesthetic elements along with the numinous. Indeed, he traces the main development of religion as successive stages of the interpenetration of the numen with rational/moral dimensions, creating a unified fabric in which the warp (rational/moral) and the woof (numen) are intertwined. On this base, he suggests as a criterion of religions the extent to which they hold these elements together in harmony. "The degree in which both rational and non-rational elements are jointly present, united in healthy and lovely harmony, affords a criterion to measure the relative rank of religions." This understanding of religion enables Otto to be open to the validity of all religions while holding to the supremacy of Christianity in bringing to mature actuality what is potential in every religion.
Otto devoted the intellectual efforts of his later years to show how the numinous and reason and morality are positively and essentially conjoined. He did this in two major ways: by showing how the complex qualities of the Holy are manifest in Christianity (as in Aufsätze das Numinose betreffend  and in Reich Gottes und Menschensohn  and in Asian religions and their relation to Christianity (as in West-Östliche Mystik  and in Die Gradenreligion Indiens und das Christentum  and by writing an imposing system of religious (Christian) ethics which he planned to use for the Gifford Lectures at Aberdeen in 1933 under the title "Moral Law and the Will of God" (Sittengesetz and Gotteswille).
Although Otto was not strong enough to deliver the lectures, it is clear that he intended to use the substance of several essays published in different journals for these Gifford lectures. In these ethical essays and his lectures on Christian ethics, Otto showed the inextricable connection between value, personal dignity, and the Holy; between the being of God which places value in and on everything created and the will of God which obligates every person to acknowledge, seek, and preserve value. Hence, there is a divine presence which obligates persons to affirm the value of all things and all persons and thereby to achieve the "dignity" of spontaneously affirming the manifestation of the Holy throughout the creation. Value acknowledgement is the way to God, who alone is altogether Holy, and God supports and sustains value in all things which bear the traces of Holiness. God's will for our salvation includes God's will that we be moral, but salvation is not restricted to morality.
In addition to his teaching Otto started three other kinds of movements: an experimental Christian liturgical community in Marburg, a museum of religious artifacts, and the Inter-religious League. In an essay, "Towards a Liturgical Reform," in his book Religious Essays, Otto shows how Holiness, taken seriously, would affect the from of the liturgy. The museum which Otto started with artifacts brought back from his travels in the East is still in Marburg. It is called the Religionskundliche Sammlung and is open to visitors along with the Rudolf Otto Archive of the University Library at Marburg. Unfortunately, the religious league is no longer in existence, but Otto's vision for it entailed not an administrative union of religions but the joining together of all religions for moral causes which each religion sustains in its own way. Otto hoped that such a league would unite persons of principle everywhere, "that the law of justice and the feeling of mutual responsibility may hold sway in the relationship between nations, races, and classes, and that the great collective moral tasks facing cultured humanity may be achieved through a closely-knit co-operation." Otto defined some of these common tasks as resisting human exploitation, upholding the position of women and of labor, and solving the problem of race. He called on the religions to become "advocates of religious, national, and social minorities against the force of the existing powers, against the arbitrary victor or the desire for revenge, against oppression and economic slavery, against world banditry and calumniation."
The failure of the league in no way dims the brilliance of Otto's religious and ethical vision nor the relevance of that vision to the way in which different religious groups confront the rational, moral, aesthetic, and religious challenges of contemporary culture. Otto was quite aware of the threat of Nazism and of other forms of brutalization and manipulation of the human spirit. Those things did not shake the central assurance of his life and work, expressed in the words chiseled on his tombstone in the cemetery at Marburg:
"Holy, holy, holy is the Lord of hosts: the whole earth is full of his glory" (Isaiah 6:3)
Most of the writing about Rudolf Otto has been in German, but there are significant essays and a few books in English. Otto influenced Joachim Wach, James Luther Adams, Paul Tillich, Mircea Eliade, Bernard Meland, and David Tracy in significant ways. Wach shows his appreciation of Otto in the essay "Rudolf Otto and the Idea of the Holy" in his book Types of Religious Experience (1951). Bernard Meland has a brilliant essay on Otto in A Handbook of Christian Theologians, edited by D. G. Peerman and M. E. Marty (1965). John M. Moore considers Otto along with William James and Henri Bergson in his book Theories of Religious Experience (1938). John Reeder emphasized Otto's ethics in an essay, "The Relation of the Moral to the Numinous in Otto's Notion of the Holy" in Religion and Morality (1973), edited by G. Outka and J. P. Reeder.
There are two excellent books on Otto's life and work in English. Robert F. Davidson published Rudolf Otto's Interpretation of Religion in 1947, and this has been an indispensable introduction of Otto to American readers. More recently, Philip C. Almond has written Rudolf Otto, An Introduction to his Philosophical Theology (1984). Both of these works identify influences on Otto and present a critical exposition of his thought. Neither one, however, treats adequately the Christian theology and Christian ethics which engaged Otto in the last years of his life.