C.S. Lewis’s autobiography about his journey from childhood faith to Christianity is Surprised by Joy.
Surprised by Joy: The Shape of My Early Life was published in 1955. Lewis describes his childhood, education, literary influences, service in World War I, intellectual friendships, and gradual movement back toward Christian belief. The book ends with his conversion story, though its emphasis is on the experiences and ideas that prepared him for that change.
The title refers to Lewis’s use of “joy” for an intense, fleeting longing triggered by imagination, literature, nature, or beauty. He distinguishes this experience from ordinary happiness and treats it as an important clue in his spiritual and intellectual development.
The autobiography is sometimes confused with A Grief Observed, which concerns his later mourning after the death of his wife, Joy Davidman. Surprised by Joy focuses primarily on Lewis’s early life and religious development.