Paradoxically, the end of “established Christianity” in the old sense resulted in the most rapid and widespread expansion in the history of Christianity. The Christianization of the Americas and the evangelization of Asia, Africa, and Australasia gave geographic substance to the Christian title “ecumenical.” Growth in areas and in numbers, however, need not be equivalent to growth in influence. Despite its continuing strength throughout the modern period, Christianity retreated on many fronts and lost much of its prestige and authority both politically and intellectually.

During the formative period of modern Western history, roughly from the beginning of the 16th to the middle of the 18th century, Christianity participated in many of the movements of cultural and political expansion. The explorers of the New World were followed closely by missionaries—that is, when the two were not in fact identical. Protestant and Roman Catholic clergymen were prominent in politics, letters, and science. Although the rationalism of the Enlightenment alienated many people from the Christian faith, especially among the intellectuals of the 17th and 18th centuries, those who were alienated often kept a loyalty to the figure of Jesus or to the teachings of the Bible even when they broke with traditional forms of Christian doctrine and life. Citing the theological conflicts of the Reformation and the political conflicts that followed upon these as evidence of the dangers of religious intolerance, representatives of the Enlightenment gradually introduced disestablishment, toleration, and religious liberty into most Western countries. In this movement they were joined by Christian individuals and groups that advocated religious freedom not out of indifference to dogmatic truth but out of a concern for the free decision of personal faith.

The state of Christian faith and life within the churches during the 17th and 18th centuries both reflected and resisted the spirit of the time. Even though the Protestant Reformation had absorbed some of the reform energy within Roman Catholicism, the theology and morals underwent serious revision in the Roman Catholic Counter-Reformation. Fighting off the attempts by various countries to establish national Catholic churches, the papacy sought to learn from the history of its encounter with the Reformation and to avoid the mistakes that had been made then. Protestantism in turn discovered that separation from Rome did not necessarily inoculate it against many of the trends that it had denounced in Roman Catholicism. Orthodox theology of the 17th century both in Lutheranism and in the Reformed churches displayed many features of medieval Scholasticism, despite the attacks of the reformers upon the latter. Partly as a compensation for the overemphasis of orthodoxy upon doctrine at the expense of morals, Pietism summoned Protestant believers to greater seriousness of personal faith and practical living.

In alliance with the spirit of the Enlightenment, the so-called “democratic” revolutions of the 18th, 19th, and 20th centuries aided this process of undermining Christianity. Roman Catholicism in France, Eastern Orthodoxy in Russia, and Protestantism in former European colonies in Africa and Asia were identified—by their enemies if not also by themselves—as part of the ancien régime and were nearly swept away with it. As the discoveries of science proceeded, they clashed with old and cherished notions about the doctrine of creation, many of which were passionately supported by various leaders of organized Christianity. The age of the revolutions—political, economic, technological, intellectual—was an age of crisis for Christianity. The critical methods of modern scholarship, despite their frequent attacks upon traditional Christian ideas, helped to produce editions of the chief documents of the Christian faith—the Bible and the writings of the Fathers and reformers—and to arouse an unprecedented interest in the history of the church. The 19th century was called the great century in the history of Christian missions, both Roman Catholic and Protestant. By the very force of their attacks upon Christianity, the critics of the church helped to arouse within the church new apologists for the faith, who creatively reinterpreted it in relation to the new philosophy and science of the modern period. The 20th century saw additional challenges to the Christian cause in the form of totalitarianism, of resurgent world religions, and of indifference. Both the relation of church and state and the missionary program of the churches thus demanded reconsideration. But the 20th century also saw renewed efforts to heal the schisms within Christendom. The ecumenical movement began within Protestantism and Anglicanism, eventually included some of the Eastern Orthodox churches, and, especially since the Second Vatican Council (1962–65), engaged the sympathetic attention of Roman Catholicism as well.