In 1942, during the darkest days of World War II, a Oxford professor took to the BBC airwaves with an unlikely message. While bombs fell on London, C.S. Lewis made the case that there was something worth believing in—and that the Christian faith could withstand the toughest intellectual scrutiny.
Those radio broadcasts became Mere Christianity, one of the most influential books of the 20th century. Eighty years later, Lewis’s arguments remain remarkably relevant. In an age of skepticism, relativism, and what some call a “post-truth” culture, his clear-eyed reasoning offers something rare: honest answers to honest questions.
This guide explores the core principles of Lewis’s apologetics, examines the three most common objections to the Christian faith, and shows how Lewis—along with contemporary voices like Frank Turek and William Lane Craig—addresses them.
Table of Contents
- Part One: The Core Principles of Mere Christianity
- Objection #1: The Problem of Evil and Suffering
- Objection #2: “All Morality Is Relative”
- Objection #3: “Jesus Was Just a Good Moral Teacher”
- Why This Still Matters
Part One: The Core Principles of Mere Christianity
The Moral Law Argument
Lewis begins not with the Bible or religious experience, but with something everyone recognizes: the sense that some things are genuinely right and others genuinely wrong.
When two people argue, Lewis observes, they don’t just express preferences—they appeal to a standard both parties acknowledge. “You promised!” “That’s not fair!” “How would you like it if someone did that to you?” These appeals only make sense if there’s an objective standard of behavior that exists outside of personal opinion.
Lewis puts it this way in Mere Christianity:
“First, that human beings, all over the earth, have this curious idea that they ought to behave in a certain way, and cannot really get rid of it. Secondly, that they do not in fact behave in that way. They know the Law of Nature; they break it. These two facts are the foundation of all clear thinking about ourselves and the universe we live in.”
This “Moral Law” is different from the laws of physics. As Lewis explains, gravity describes what happens; the Moral Law describes what ought to happen. You can’t break the law of gravity—but you can break the Moral Law, which is precisely why you feel guilt when you do.
The argument proceeds: If there’s an objective moral standard that transcends human opinion, where did it come from? A moral law implies a moral lawgiver. Lewis argues this lawgiver is what we mean when we say “God.”
“It seems, then, we are forced to believe in a real Right and Wrong. People may be sometimes mistaken about them, just as people sometimes get their sums wrong, but they are not a matter of mere taste and opinion any more than the multiplication table.”
The Trilemma: Lord, Liar, or Lunatic
Lewis’s most famous argument addresses a common claim: “I’m happy to accept Jesus as a great moral teacher, but I don’t believe he was God.”
Lewis argues this position is logically untenable. Jesus made explicit claims to divinity—forgiving sins, accepting worship, claiming unity with the Father. A merely human teacher who made such claims would not be “great” at all:
“I am trying here to prevent anyone saying the really foolish thing that people often say about Him: ‘I’m ready to accept Jesus as a great moral teacher, but I don’t accept His claim to be God.’ That is the one thing we must not say. A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—on the level with the man who says he is a poached egg—or else he would be the Devil of Hell.”
The trilemma forces a decision:
- Liar: Jesus knew his claims were false and deliberately deceived people
- Lunatic: Jesus sincerely believed his claims but was delusional
- Lord: Jesus’s claims were true—he really was who he said he was
Lewis concludes: “You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse.”
Philosopher Peter Kreeft calls this “the most important argument in Christian apologetics.”
What “Mere Christianity” Actually Means
Lewis wasn’t defending Catholicism, Anglicanism, or any particular denomination. He was defending what he called “mere” Christianity—the core beliefs shared by virtually all Christians across time and tradition:
- God exists and created the universe
- Humans have rebelled against God (sin)
- Jesus Christ is both fully God and fully human
- Through his death and resurrection, Jesus provides the way for humans to be reconciled to God
- This reconciliation transforms us from the inside out
Lewis compared Christianity to a great hall with many rooms (denominations) opening off it. His goal was to bring people into the hall; which room they entered was between them and God.
“In religion, as in war and everything else, comfort is the one thing you cannot get by looking for it. If you look for truth, you may find comfort in the end: if you look for comfort you will not get either comfort or truth—only soft soap and wishful thinking to begin with and, in the end, despair.”
Objection #1: The Problem of Evil and Suffering
The Objection
This is the objection most people find compelling: If God is all-powerful and all-good, why does evil exist? Why do innocent children suffer? Why do natural disasters kill thousands? Either God can’t stop evil (and isn’t all-powerful) or won’t stop evil (and isn’t all-good).
The ancient philosopher Epicurus framed it this way: “Is God willing to prevent evil, but not able? Then he is not omnipotent. Is he able, but not willing? Then he is malevolent.”
Lewis himself wrestled deeply with this question. Before his conversion, the problem of evil was his greatest obstacle to faith. He remembered the quote from the Roman poet Lucretius: “Had God designed the world, it would not be / A world so frail and faulty as we see.”
How Lewis Responds
Lewis’s response, developed in The Problem of Pain, centers on free will. God created beings with genuine freedom—the ability to choose love or rejection, good or evil. This freedom is essential to any meaningful existence:
“Free will, though it makes evil possible, is also the only thing that makes possible any love or goodness or joy worth having. A world of automata—of creatures that worked like machines—would hardly be worth creating.”
Could God have created a world without the possibility of suffering? Lewis argues this request is actually incoherent:
“Try to exclude the possibility of suffering which the order of nature and the existence of free-wills involve, and you find that you have excluded life itself.”
But Lewis goes further. He argues that pain, while genuinely evil, can serve a purpose. It wakes us up:
“Pain is unmasked, unmistakable evil; every man knows that something is wrong when he is being hurt… God whispers to us in our pleasures, speaks in our conscience, but shouts in our pains: it is His megaphone to rouse a deaf world.”
Crucially, Lewis noticed something about his own argument against God. As the C.S. Lewis Institute explains: “It gradually dawned on him that his argument depended on the idea that there was, in fact, real evil in the world. Evil was not an illusion or just a feeling or emotive response to an unpleasing event. But, where had he gotten this idea of evil? He realized that his atheism provided no basis for it.”
In other words, the very ability to call something “evil” points toward an objective standard—and that standard points toward God.
Other Apologetic Voices
Contemporary philosopher William Lane Craig echoes Lewis’s insight: the problem of evil, properly understood, is actually evidence for God, not against him. If the universe is purely material with no transcendent standard, then “evil” is just a word we use for things we don’t like. There’s no objective basis for moral outrage.
Frank Turek puts it bluntly: “You cannot complain about the problem of evil if there is no God. If there is no God, then there is no such thing as good or evil… If evil exists, God exists.”
Philosopher Alvin Plantinga’s “Free Will Defense” has shown, in rigorous philosophical terms, that there is no logical contradiction between God’s existence and the existence of evil. As scholars note, “Plantinga has shown that the existence of evil in theism can never again be charged to be a necessary contradiction.”
Objection #2: “All Morality Is Relative”
The Objection
This objection takes a different approach: “Who are you to say what’s right and wrong? Morality is just cultural conditioning. What’s true for you isn’t necessarily true for me. There are no moral absolutes.”
This view—moral relativism—has become almost a default assumption in modern Western culture. To claim that something is objectively wrong seems arrogant, judgmental, even oppressive.
How Lewis Responds
Lewis argues that no one actually believes in moral relativism—not when it affects them personally. When someone cuts in line, takes your parking spot, or breaks a promise, you don’t say “Well, that’s their truth.” You appeal to a standard you expect them to recognize.
“Whenever you find a man who says he does not believe in a real Right and Wrong, you will find the same man going back on this a moment later. He may break his promise to you, but if you try breaking one to him he will be complaining ‘It’s not fair’ before you can say Jack Robinson.”
Lewis also points to the remarkable consistency of moral codes across cultures. While details vary, the core principles—don’t murder, don’t steal, honor your commitments, protect the vulnerable—appear everywhere. As LitCharts notes, Lewis uses this cross-cultural moral consensus as evidence for an objective moral standard.
The Moral Law, Lewis argues, is like a tune we’re meant to play; our instincts are merely the keys:
“The sheet of music which tells you, at a given moment, to play one note on the piano and not another, is itself not one of the notes on the keyboard. The Moral Law tells us the tune we have to play: our instincts are merely the keys.”
Frank Turek’s Approach
In I Don’t Have Enough Faith to Be an Atheist, Frank Turek and Norman Geisler extend Lewis’s argument.
They point out that the claim “there are no absolutes” is itself an absolute claim—and therefore self-refuting. Similarly, “all truth is relative” cannot be true, because if it were true, it would be an absolute truth, contradicting itself.
Turek states it simply: “There is no such thing as your truth. There is no such thing as my truth. There is just the truth.”
He also presses the moral argument to its logical conclusion: “If there’s one thing morally wrong out there, then there has to be a God because we recognize a standard.” If God doesn’t exist, he argues, then there are no human rights—only preferences. Without a transcendent moral standard, there’s no objective difference between love and abuse, generosity and theft. We may prefer one over the other, but we can’t say one is actually better.
This isn’t just theoretical. Turek points out that “all laws legislate morality”—the only question is whose morality gets legislated. Without an objective standard, moral debates reduce to power struggles.
Objection #3: “Jesus Was Just a Good Moral Teacher”
The Objection
This is perhaps the most common way people try to appreciate Jesus while avoiding the implications of his claims. “Jesus had some beautiful teachings about love and forgiveness. He was a wise man, a prophet, a moral exemplar. But he wasn’t actually God—that’s just mythology added later by his followers.”
This view allows people to admire Jesus while rejecting the supernatural elements of Christianity.
How Lewis Responds
Lewis’s trilemma (discussed above) directly addresses this objection. But he presses the point even further: look at what Jesus actually said.
Jesus didn’t just teach ethics. He forgave sins—not sins committed against him personally, but sins in general. He accepted worship. He claimed that he and the Father were one. He said that seeing him was seeing God. He claimed authority over the Sabbath, over the Temple, over life and death itself.
As Lewis puts it:
“A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would either be a lunatic—on the level with the man who says he is a poached egg—or else he would be the Devil of Hell. You must make your choice.”
The “good moral teacher” option simply isn’t available. A good moral teacher doesn’t claim to be God if he isn’t. That would be either insanity or deception—neither of which is compatible with being a “good teacher.”
The Historical Evidence
Some skeptics argue that Jesus never actually claimed divinity—that these claims were invented by later followers. But this view faces serious historical problems.
William Lane Craig points out that the earliest Christian documents (Paul’s letters, dating to within 20 years of Jesus’s death) already assume Jesus’s divine status. The Gospels, written within living memory of the events, record Jesus’s claims in detail. If the early church invented these claims, the invention happened remarkably fast and remarkably uniformly.
Furthermore, Jesus’s claims led directly to his execution. The charge against him was blasphemy—claiming divine prerogatives. His earliest followers died for the belief that he had risen from the dead and was who he claimed to be. People don’t typically die for claims they invented.
Craig’s Kalam cosmological argument provides additional grounding: if the universe had a beginning (as modern cosmology confirms), it requires a cause. That cause must be outside space and time, immaterial, and enormously powerful. This points toward the kind of God Jesus claimed to reveal.
Why This Still Matters
Lewis wrote during a time of global crisis—world war, genocide, the threat of civilization’s collapse. Today we face different challenges: information overload, ideological fragmentation, a pervasive sense that nothing is certain and everything is up for debate.
In this environment, Lewis’s approach offers something valuable: the insistence that truth matters, that reason is trustworthy, and that the biggest questions of life deserve serious investigation.
He wasn’t interested in cheap answers or emotional manipulation. He wanted to know if Christianity was true—and if so, what followed from that.
“Christianity, if false, is of no importance, and if true, of infinite importance. The only thing it cannot be is moderately important.”
Lewis’s arguments aren’t the final word. Critics have responded; debates continue. But his work demonstrates that faith and reason aren’t enemies. The Christian faith can be examined, questioned, and defended in the marketplace of ideas.
Perhaps that’s his most enduring contribution: permission to think hard about what you believe and why—and the confidence that honest inquiry, pursued with humility, leads somewhere worth going.
“Aim at Heaven and you will get earth ‘thrown in’: aim at earth and you will get neither.”
Sources
- C.S. Lewis Institute
- PBS: The Question of God – Mere Christianity
- C.S. Lewis and Mere Christianity: The Moral Law
- LitCharts: Mere Christianity Themes
- Wikipedia: Lewis’s Trilemma
- C.S. Lewis Institute: The Problem of Evil
- Goodreads: The Problem of Pain Quotes
- The Marginalian: C.S. Lewis on Free Will
- CrossExamined.org – Frank Turek
- I Don’t Have Enough Faith to Be an Atheist – Geisler & Turek
- Reasonable Faith – William Lane Craig
- The Kalam Cosmological Argument – William Lane Craig
- Trinity Community Church: Top Quotes from Mere Christianity
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