This essay, given as the first of three lectures comprising the Charles E. Test Lectures at the Madison program at Princeton in May 2017, initiates the project of defending T. S. Eliot’s suggestion that the future of Western societies will be determined by a contest between Christianity and “modern paganism.” The essay attempts to give an account of “paganism” as immanent religiosity, in contrast to the transcendent religiosity of later Judaism and Christianity. It then discusses the displacement of paganism by Christianity in late antiquity, but suggests (using Ronald Dworkin as the central example) that paganism has enjoyed a revival in recent times. Hence, our current cultural struggles are less helpfully thought of as clashes of “religion v. secularism” than as a struggle between competing forms of religiosity.