C.S. Lewis identified his conversion to Christianity as having occurred in 1931.
Lewis had returned to belief in God several years earlier, influenced partly by conversations with Christian friends and by reading authors including George MacDonald and G.K. Chesterton. He described his movement from atheism to theism and then to Christianity as a gradual intellectual and spiritual process rather than a single simple event.
In his autobiography, Surprised by Joy, Lewis wrote that his conversion to Christianity took place in 1931. He later recalled a decisive journey to Whipsnade Zoo with his brother Warren, saying that he went to the zoo as a theist and returned believing in Christ.
The date can be confused with 1929, when Lewis said he submitted to God and became a theist. Those are different stages in his account. After 1931, Christianity became the central subject of much of his nonfiction, including The Problem of Pain, Mere Christianity, and The Screwtape Letters.