Which C.S. Lewis book explains the Christian idea of the Trinity through three-dimensional geometry?

Mere Christianity explains the Christian idea of the Trinity through a three-dimensional geometry analogy.

Lewis uses the image of a two-dimensional figure becoming three-dimensional to illustrate how Christian belief in one God and three persons can exceed the limits of ordinary human experience. He develops the analogy while discussing Christian doctrine in the book’s section on theology.

Mere Christianity grew out of radio talks Lewis gave during the Second World War. The broadcasts were later revised and published in separate parts, including The Case for Christianity, Christian Behaviour, and Beyond Personality. The combined book became one of Lewis’s best-known works of Christian apologetics.

The geometry image is an analogy, not a mathematical proof of the Trinity. Lewis presents it to show why a doctrine can seem contradictory when viewed from a limited perspective. This distinction matters because readers sometimes treat the illustration as a complete theological explanation rather than as a teaching device.