The idea that religion gradually ceases to be the guiding authority in the lives of individuals and in societies as they become more modern has roots in the intellectual and institutional heritage of the Enlightenment. But even in Enlightenment thought, there was never just one understanding of the relationship between the progress of humanity and the future of religion. Only a few prophets of religion’s decline—Karl Marx being the most notable among them—dared to predict that the world of the future would be a world without religion. Others, like Thomas Jefferson, did not speak of, or look forward to, the end of religion as such, but predicted that human enlightenment would be accompanied by a rational form of religious knowledge and experience. The history of the idea of secularization, in other words, has been a complex one and has involved a number of different, nuanced views.