C.S. Lewis converted to Christianity in 1931 after a period in which he described himself as an atheist.
Lewis's return to Christianity developed over several years rather than through one isolated event. He had moved from childhood belief to atheism during adolescence, then gradually reconsidered his views while studying literature, philosophy, and mythology. His conversations with Christian friends, including J.R.R. Tolkien and Hugo Dyson, influenced that process.
Lewis became a theist in 1930 and then embraced Christianity in 1931. He later described this change in autobiographical works such as Surprised by Joy. His conversion shaped nearly all of his later public writing, including both apologetic books and the Narnia stories.
A common oversimplification places every stage of his religious change in 1931. The year is used for his conversion to Christianity, while his earlier movement away from atheism toward belief in God occurred before that.