Historians differ as to exactly when modernity began. One possibility is c1400, in the Renaissance, which supposedly marked the beginnings of individualism and humanism. Another date is 1500 -- the coincidence of the geographical expansion of Europe and the sundering of the spiritual unity of the West by the Protestant Reformation. Next is 1600, triggered by the New Science which soon became the dominant influence in Western thought. Finally, around 1800, came both the Industrial Revolution, which transformed the lives of ordinary people all over the world in fundamental and lasting ways, and the advent of democracy as the normative modern political system.
Modernity in the West is in fact a synthesis of all these elements, and more besides, each of which helped transform Western civilization in unique ways, making it quite different not only from the civilizations of the rest of the world, all of which eventually came under Western influence, but also quite different from Western civilization itself in the pre-modern era.
Thus a key element of modernity is precisely the sense that modern Westerners have that they are living in a new age. In the Renaissance, for example, artists often portrayed scenes from ancient times -- the Bible, Greece, Rome -- with their subjects dressed like Italians of the fifteenth century. Apparently there was little sense of how different the past was from the present. But it was during the Renaissance also that this outlook began to change, and one of the major manifestations of modernity is what is called historical consciousness -- an acute awareness of change and of how each age must be understood in its unique context.
Taken to extreme, historical consciousness leads to what is called historicism -- the conviction that everything is so conditioned by its age that there is no truth which transcends temporal change. Historicism, however, did not develop until relatively late, essentially during the nineteenth century.
The radical nature of the Renaissance has often been exaggerated. Most of the artists and thinkers of the time were still believing Catholics, many of them quite devout. They did, however, tend to reject Scholasticism, the Church's dominant mode of theologizing (St. Thomas Aquinas, most notably), as too abstract and technical, irrelevant to the man-centered perspective which the Renaissance humanists expounded. In that sense they opened an intellectual wedge between themselves and the medieval past. Renaissance humanism by no means repudiated religion, but it widened the scope of both artistic and intellectual creativity to include the celebration of human achievement, and in that sense it helped prepare the way for later secularization.
The full implications of the Age of Discovery were not realized for a long time. But the European explorers who began sailing the globe after 1400 permanently shattered the enclosed self-sufficiency of the various world civilizations, including the West, a process which continues unabated to the present. Sooner or later, in a variety of ways, Westerners came to relativize their own culture, including their religious beliefs, in the light of the beliefs and practices of the other cultures of the world.
So also the Protestant Reformers did not think of themselves as revolutionaries but as devoted to restoring the authentic past, the Christianity of the New Testament. But this drove a still deeper wedge between themselves and the past, casting into doubt the Christianity of the previous 1200 or more years. The unity of Christendom was shattered, the unintended effect of the Reformation being to weaken Christianity's intellectual and social influence, in part through the inevitable human tendency to assume that, if people sharply disagree about the truth, perhaps none of them in fact posses it.
As a result of the weakening and fragmentation of Christianity, religion ceased to be the dominant, essentially unquestioned, source of spiritual and moral authority in the West. Among other things the way was prepared for the triumph of the secular state in ways which subordinated religion to political purposes.
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