The Idea of the Holy: An Inquiry into the Non-Rational Factor in the Idea of the Divine and Its Relation to the Rational by Rudolf Otto is a seminal work in the philosophy of religion and theology. The book introduces the concept of the "numinous" as a way to describe the profound, non-rational experience of the holy that transcends intellectual understanding. Otto's work has had a significant impact on the study of religion and remains a cornerstone in understanding the nature of religious experience. Otto's work remains a timeless exploration of the mysterious and transcendent, offering profound insights into the nature of the divine and humanity's response to it.