Religion serves for millions of people as a substitute for philosophy. It serves as the guide of their private and public lives, and impinges, sometimes powerfully, upon the lives of their non-religious fellow citizens. The last few decades have witnessed a world-wide revival of forms of religion undiluted by the influences of the Enlightenment period. In the Western World, the main example of this regressive trend is a movement in Protestant Christianity known as Fundamentalism. These two lectures are concerned with the origins and nature of this religious ideology, its sense of life, its deep psychological appeal, and its threat to the lives of non-religious people.