Which C.S. Lewis book develops the idea that Christianity is like a hall with many doors?

C.S. Lewis develops the idea of Christianity as a hall with many doors in Mere Christianity.

In Mere Christianity, Lewis compares the shared core of Christian belief to a hallway, with different denominations represented by doors leading into rooms. He presents the image to distinguish common Christian doctrine from the practices and structures of individual churches. The analogy appears in the book’s discussion of how readers should move from broad agreement toward a particular Christian tradition.

Mere Christianity grew out of radio talks Lewis gave for the BBC during the Second World War. The talks were revised and published in book form, first in separate sections and later as a single volume. The hall-and-doors image is often remembered as a general statement about denominational choice, but Lewis also stresses that the rooms themselves matter.