C.S. Lewis’s conversion to Christianity is generally dated to 1931.
Lewis had returned to belief in God before that year, influenced by conversations with friends and fellow writers. His movement from atheism toward Christianity developed gradually rather than through a single simple event. He later described himself as becoming a theist and then, in 1931, accepting Christianity.
Important influences included J.R.R. Tolkien and Hugo Dyson, with whom Lewis discussed myth, imagination, and Christian belief. Lewis’s conversion transformed the direction of his writing. He went on to publish apologetic works such as The Problem of Pain, Mere Christianity, and Miracles, while also expressing Christian themes through fiction.
Biographies sometimes distinguish between Lewis’s return to belief in God in the late 1920s and his conversion to Christianity in 1931. That distinction explains why more than one date may appear in summaries. For the specifically Christian conversion, 1931 is the standard answer.