C.S. Lewis became a Christian in 1931 after an important period of conversion.
Lewis had returned to belief in God by 1929, but his movement toward Christianity continued through conversations and friendships with figures including J.R.R. Tolkien and Hugo Dyson. In September 1931, Lewis described himself as becoming a Christian after a late-night conversation with Tolkien and Dyson at Oxford. His conversion shaped nearly all of his later public writing.
Lewis had been raised in the Church of Ireland but became an atheist during adolescence. His return to Christianity was therefore a major change in his intellectual and personal life, not simply a continuation of childhood practice.
The precise stages are sometimes compressed into one date. Lewis’s own accounts distinguish between his return to theism in 1929 and his conversion to Christianity in 1931, making 1931 the standard answer for the Christian conversion.