The concept of secularization has grown to become one of the most important features of contemporary religious though. This book introduces and examines the thinking of sixteen key theologians, philosophers and historians of religion to explain (a) why by the late nineteenth century the traditional concept of God as an ontologically real being came to be considered no longer necessary and (b) how the new perspective on God, which accepts him only as an idea, turned into the preferred approach of today's religion and philosophy, namely religious radicalism. While it affects both philosophy and theology, religious radicalism sets out to present the idea of God as applicable to the human being in all respects, to the point that the word God becomes a mirror for the acute, rational, and experiential awareness of contemporary men and women. Readers will find this book an insightful and useful guide to contemporary thinking on secularization in the religious and theological context.