C.S. Lewis became a Christian in 1931 after years of intellectual and spiritual change.
Lewis had moved away from the Christianity of his childhood and described himself for a period as an atheist. His friendships and conversations with Christian writers, especially J.R.R. Tolkien and Hugo Dyson, contributed to his reconsideration of Christian belief. Lewis later described his conversion as a gradual process rather than a single simple event.
He first returned to belief in God in 1930 and became a Christian in 1931. His conversion profoundly shaped his later writing, including The Problem of Pain, Mere Christianity, and The Screwtape Letters. He also became a prominent defender of Christianity through books, essays, lectures, and radio broadcasts.
Accounts sometimes distinguish between Lewis’s return to theism and his later acceptance of Christianity. That distinction explains why different dates may appear in summaries. For the question asking when he became a Christian, 1931 is the standard answer.