C.S. Lewis became a Christian in 1931, after a period of intellectual and spiritual change.
Lewis had returned to belief in God in 1929, but his conversion to Christianity followed in 1931. He later described important conversations with J.R.R. Tolkien and Hugo Dyson, especially during a walk and late-night discussion in September of that year. Lewis's account of this development appears in autobiographical and biographical sources, although the details are interpreted differently by scholars.
His conversion transformed the direction of his writing. He became one of the twentieth century's most influential Christian apologists, producing works such as The Problem of Pain, Mere Christianity, and Miracles. His fiction also frequently engages with moral, theological, and spiritual themes.
A common mistake is to give 1929 as the date of his Christian conversion. That year marks Lewis's return to belief in God, while 1931 is generally given for his conversion to Christianity. The distinction matters because Lewis described the two stages separately.