C. S. Lewis' conversion to Christianity has been recounted many times but rarely through the lens of his evolving philosophy. C. S. Lewis and Idealism traces the lesser-known intellectual journey that led Lewis from realism to idealism, and ultimately to theism and the Christian faith. Drawing on Lewis's early writings and the influences of George Berkeley, Owen Barfield, and, surprisingly enough, Rudolf Steiner, this study explores how philosophical idealism shaped Lewis' understanding of reality and the supernatural. It distinguishes between his adherence to Absolute idealism, which he called "philosophical idealism," and the more solipsistic ideas he later developed, revealing how these shifts paved the way for his theistic worldview. By re-examining the steps Lewis took both due to reason and revelation, C. S. Lewis and Idealism offers fresh insight into the mind behind the books Mere Christianity, Miracles, and Surprised by Joy. It invites readers to see Lewis not only as a beloved apologist, but as a thinker whose spiritual awakening was deeply rooted in philosophy.