Neo-orthodoxy

Neo-orthodoxy or Neoorthodoxy, in Christianity, 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. 
A similar title has been given to the unrelated Eastern Orthodox theology of Christos Yannaras, John Zizioulas and John Romanides.
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". Instead, under pseudonyms such as Johannes Climacus, Kierkegaard maintained that Christianity is "absurd" (i.e., it transcends human understanding) and presents the individual with paradoxical choices. The decision to become a Christian, Kierkegaard thought, is not fundamentally rational but passional—a leap of faith. Opponents of Kierkegaard's approach and neo-orthodoxy in general have termed this fideism, a blatant refusal to find support for the faith outside its own circles. For the most part, proponents reply that no such support exists, that supposed reasons and evidences for faith are fabrications of fallen human imagination, and in effect constitute idolatry. Some neo-orthodox proponents have gone so far as to claim greater affinity with atheists in that regard than with the theological and cultural trappings of so-called "Christendom", which Kierkegaard venomously denounced in his later works. (Dietrich Bonhoeffer's "religionless Christianity" and the later secular theology reflect similar conclusions.)
Sin and human nature.
In neo-orthodoxy, sin is seen not as mere error or ignorance; it is not something that can be overcome by reason or social institutions (e.g., schools); it can only be overcome by the grace of God through Jesus Christ. Sin is seen as something bad within human nature itself. This amounts to a renovation of historical teachings about original sin (especially drawing upon Augustine of Hippo), although thinkers generally avoided forensic interpretations of it and consequential elaborations about total depravity. The means of supposed transmission of sin, to neo-orthodox minds, is not as important as its pervasive reality. The association of original sin with sexuality—an abiding idea—leads to moralism, a rectitude that is overly optimistic about human capabilities to resist the power of unfaith and disobedience in all areas of life, not just sexual behavior. This core conviction about the universality and intransigence of sin has elements of determinism, and has not surprisingly offended those who think people are capable, wholly or in part, of effecting their own salvation (i.e., synergism). In other words, neo-orthodoxy might be said to have a greater appreciation of tragedy in human existence than either conservatism or liberalism, a point emphasized by a latter-day interpreter of the movement, Canadian theologian Douglas John Hall.
Relation to other theologies.
Neo-orthodoxy is distinct from both liberal Protestantism and evangelicalism, but, notwithstanding some interpreters, it cannot properly be considered a mediating position between the two. Neo-orthodoxy draws from various Protestant heritages (primarily Lutheran and Calvinist) in an attempt to rehabilitate dogma outside the restraints of Enlightenment thought. Unlike confessionalist or fundamentalist reactions to individualist approaches to the faith, however, neo-orthodox adherents saw no value in rehabilitating tradition for its own sake. Past Protestant doctrine is used only to the degree that it affirms the living Word of God in Jesus Christ. Propositions in and of themselves, whether from the Bible or not, are, to the neo-orthodox, insufficient to build theology upon. Also, in the pursuit of social justice, intellectual freedom, and honesty, the neo-orthodox, unlike the conservatives they were accused by detractors of resembling, often made practical alliances with liberals. Both groups shared a deep hostility to authoritarianism of any kind, in both church and state.
The breadth of the term "neo-orthodox", though, has led to its abandonment as a useful classification, especially after new emphases in mainline Protestant theology appeared during the 1960s. These included the "Death of God" movement, which attacked the linguistic and cultural foundations of all previous theology, and a renewal of interest among Biblical scholars in the "historical Jesus", something neo-orthodox theologians largely dismissed as irrelevant to serious Christian faith. Still, some of the movement's positions and worldviews would inform such later movements as liberation theology during the 1970s and 1980s and post-liberalism during the 1990s and 2000s—in spite of theological and ethical differences from both (i.e., liberationist use of Marxist conceptual analysis and narrativist dependence upon virtue theory).
Influence upon American Protestantism.
From its inception, this school of thought has largely been unacceptable to Protestant evangelicalism, as neo-orthodoxy generally accepts biblical criticism; has remained mostly silent on the perceived conflicts caused by evolutionary science; and, in espousing these two viewpoints, it retains at least some aspects of 19th-century liberal theology.
Recent critical scholarship.
While some German scholars since the 1990s have warned English-speaking scholars against a too-serious application of neo-orthodoxy—calling it a misreading of the writing of Karl Barth, who, with his predecessors and contemporaries, should be understood in terms of historical forces—the fact is that neo-orthodoxy was and remains a valid method of scholarship.