Which C.S. Lewis book was his first published work after converting to Christianity?

C.S. Lewis’s first published work after converting to Christianity was The Pilgrim’s Regress.

Published in 1933, The Pilgrim’s Regress is an allegorical novel subtitled An Allegorical Apology for Christianity, Reason and Romanticism. Its central character, John, travels through a landscape filled with figures and institutions representing intellectual and spiritual experiences. The title deliberately echoes John Bunyan’s The Pilgrim’s Progress.

Lewis had returned to Christian belief in stages, becoming a theist in 1930 and embracing Christianity in 1931. The Pilgrim’s Regress was the first major book he published after that conversion. Some readers instead name The Problem of Pain, published in 1940, because it was his first widely prominent Christian apologetic book. That later work is non-fiction; The Pilgrim’s Regress is an allegorical narrative.