In Christianity, Neo-orthodoxy or Neoorthodoxy, also known as theology of crisis and dialectical theology, was a theological movement developed in the aftermath of the First World War. The movement was largely a reaction against doctrines of 19th-century liberal theology and a reevaluation of the teachings of the Reformation. Karl Barth is the leading figure associated with the movement. In the U.S., Reinhold Niebuhr was a leading exponent of neo-orthodoxy. It is unrelated to Eastern Orthodoxy. Revelation Neo-orthodoxy strongly emphasises the revelation of God by God as the source of Christian doctrine. This is in contrast to natural theology, whose proponents include Thomas Aquinas, who states that knowledge of God can be gained through a combination of observation of nature and human reason; the issue remains a controversial topic within some circles of Christianity to this day. Barth totally rejects natural theology. As Thomas Torrance wrote: Emil Brunner, on the other hand, believed that natural theology still had an important, although not decisive, role. This led to a sharp disagreement between the two men, the first of several controversies that prevented the movement from acquiring a unified, homogeneous character. Transcendence of God Most neo-orthodox thinkers stressed the transcendence of God. Barth believed that the emphasis on the immanence of God had led human beings to imagine God to amount to nothing more than humanity writ large. He stressed the "infinite qualitative distinction" between the human and the divine, a reversion to older Protestant teachings on the nature of God and a rebuttal against the intellectual heritage of philosophical idealism. This led to a general devaluation of philosophical and metaphysical approaches to the faith, although some thinkers, notably Paul Tillich, attempted a median course between strict transcendence and ontological analysis of the human condition, a stand that caused a further division in the movement. Existentialism Some of the neo-orthodox theologians made use of existentialism. Rudolf Bultmann (who was associated with Barth and Brunner in the 1920s in particular) was strongly influenced by his former colleague at Marburg, the German existentialist philosopher Martin Heidegger. Reinhold Niebuhr and (to a lesser extent, and mostly in his earlier writings) Karl Barth, on the other hand, were influenced by the writings of the 19th-century Danish philosopher Søren Kierkegaard. Kierkegaard was a critic of the then-fashionable liberal Christian modernist effort to "rationalise" Christianity—to make it palatable to those whom Friedrich Schleiermacher termed the "cultured despisers of religion".