C.S. Lewis's conversion to Christianity is conventionally dated to 1931.
Lewis had returned to belief in God by 1929, but he later described a further conversion to Christianity in 1931. He credited conversations with friends, including J.R.R. Tolkien and Hugo Dyson, with helping him think through the idea that Christianity could be understood as a true myth. Lewis became a member of the Church of England.
His conversion did not instantly produce all of his later books, but it shaped his work as a Christian apologist, novelist, and literary scholar. Mere Christianity, The Problem of Pain, and The Chronicles of Narnia all emerged later. The dates 1929 and 1931 are both relevant to Lewis’s spiritual history: 1929 marks his return to belief in God, while 1931 marks his stated conversion to Christianity.