Though philosophers such as Nietzsche, Wittgenstein, and Sartre all argued that religion had little to teach people in the modern age, the decades after the First World War witnessed a tenacious revival of Christian thought. Christianity — and religion in general — had been on the defensive in intellectual circles since the Enlightenment. In the years before 1914 some theologians, especially Protestant ones, had felt the need to interpret Christian doctrine and the Bible so that they did not seem to contradict science, evolution, and common sense. They saw Christ primarily as a great moral teacher and downplayed the mysterious, spiritual aspects of his divinity. Indeed, some modern theologians were embarrassed by the miraculous, unscientific aspects of Christianity and rejected them.

Especially after World War I, a number of thinkers and theologians began to revitalize the fundamental beliefs of Christianity. Sometimes called Christian existentialists because they shared the loneliness and despair of atheistic existentialists, they stressed human beings’ sinful nature, their need for faith, and the mystery of God’s forgiveness. The revival of Christian belief after World War I was fed by the rediscovery of the work of the nineteenth-century Danish theologian Søren Kierkegaard (KIHR-kuh-gahrd), whose ideas became extremely influential. Kierkegaard believed it was impossible for ordinary individuals to prove the existence of God, but he rejected the notion that Christianity was an empty practice. In his classic Sickness unto Death (1849), Kierkegaard mastered his religious doubts by suggesting that people must take a “leap of faith” and accept the existence of an objectively unknowable but nonetheless awesome and majestic God.

In the 1920s the Swiss Protestant theologian Karl Barth (1886–1968) propounded similar ideas. In brilliant and influential writings, Barth argued that human beings were imperfect, sinful creatures whose reason and will are hopelessly flawed. Religious truth is therefore made known to human beings only through God’s grace, not through reason. People have to accept God’s word and the supernatural revelation of Jesus Christ with awe, trust, and obedience, not reason or logic.

Among Catholics, the leading existential Christian was Gabriel Marcel (1889–1973). Born into a cultivated French family, Marcel found in the Catholic Church an answer to what he called the postwar “broken world.” Catholicism and religious belief provided the hope, humanity, honesty, and piety for which he hungered. Marcel and his countryman Jacques Maritain (1882–1973) denounced anti-Semitism and supported closer ties with non-Catholics.

After 1914 religion became much more meaningful to intellectuals than it had been before the war. Between about 1920 and 1950, in addition to Marcel and Maritain, poets T. S. Eliot and W. H. Auden, novelists Evelyn Waugh and Aldous Huxley, historian Arnold Toynbee, writer C. S. Lewis, psychoanalyst Karl Stern, and physicist Max Planck all either converted to a faith or became attracted to religion for the first time. Religion, often of a despairing, existential variety, was one meaningful answer to uncertainty and anxiety and the horrific costs of the First and Second World Wars. In the words of English novelist Graham Greene, a Roman Catholic convert, “One began to believe in heaven because one believed in hell.”2