Which Christian doctrine does C.S. Lewis explain through the trilemma in Mere Christianity?

C.S. Lewis’s trilemma in Mere Christianity addresses the divinity of Jesus.

Lewis presented an argument about Jesus’s claim to authority, often summarized by the phrase “Lord, liar, or lunatic.” The argument says that if Jesus made certain claims about himself, readers should not simply treat him as a great moral teacher while rejecting the implications of those claims.

The trilemma appears in Lewis’s broader Christian apologetic work, which developed from radio talks broadcast by the BBC during World War II. Those talks were later revised and published in book form as Mere Christianity. The work aimed to explain shared Christian beliefs rather than defend one denomination.

The argument is frequently shortened into a slogan and sometimes expanded into “Lord, liar, or lunatic.” Lewis’s original discussion is more detailed than the slogan suggests, and critics have debated its assumptions. Its subject, however, is specifically how to understand Jesus’s identity and claims.