Mere Christianity by Clive Staples Lewis (more commonly known as C. S. Lewis) was first published in 1952 as an expansion of some radio talks Lewis gave during World War II. Though Lewis is best known for his children’s fantasy series The Chronicles of Narnia (particularly The Lion, the Witch and the Wardrobe), Mere Christianity is likely Lewis’s most famous work of Christian apologetics—a genre dedicated to addressing various critiques of Christian theology.

Lewis was well poised to make this kind of argument, having grown disillusioned with Christianity as a teenager only to return to it as an adult. The success Mere Christianity has enjoyed since its publication is also due to its accessibility; Lewis was a scholar of literature rather than of theology, so he discusses complicated religious concepts in more conversational terms than a non-layperson might. Other works of apologetics by Lewis include The Problem of Pain (1940) and The Screwtape Letters (1942).

This study guide refers to the 2001 HarperCollins edition of the work.

Summary

At the book’s outset, Lewis states that that there are aspects of Christian thought that have become muddled and that Christians themselves have been subject to internal strife. Lewis seeks to restore unity to the Christian religion, focusing on the difference between Christian and non-Christian belief (as opposed to disputes between—and within—the various denominations of Christianity).

Lewis begins by discussing morality, arguing that almost all humans have an innate sense of right and wrong and that the content of this moral code is largely universal. Although Lewis acknowledges that cultural differences do exist, he believes that these are generally minor and superficial. However, while this moral law appears to be objective in a certain sense, it isn’t binding; human beings have free will and can disobey it. Lewis concludes Book 1 by suggesting that while only a force similar to our own mind could provide us with a sense of what is good and right, our own behavior must often put us at odds with that force.

In Book 2, Lewis considers various religious ideas of what this force might be in light of his earlier discussion of the existence of good and evil. Whereas Pantheists believe that God is the universe, Christianity believes that God created the universe. It follows that, for Pantheists, God is both good and bad—or rather, that our understanding of good and bad is the byproduct of our own limitations and that God is beyond such concepts. For Christians, by contrast, God is infinitely good and wants humans to behave in particular ways. 

Although Christianity recognizes that people can be wicked, it does not see badness as inherent in the way that religious Dualism does; to the Christian, all badness is ultimately perverted goodness, twisted as a result of humanity’s fall, which was the result of people thinking they could find happiness outside of God. The Christian story is ultimately about how the Son of God (Jesus Christ) took humanity’s sins upon Himself because only God could do “perfect” penance for those sins and, in the process, restore us to our original nature. It is up to us, however, to choose the life that Christ’s sacrifice offers to us.

Book 3 elaborates on what that choice looks like in practice, expanding on the three “Theological” virtues (faith, hope, and charity) and the four “Cardinal” virtues (prudence, temperance, justice, and fortitude) that Christians should seek to practice. Lewis also devotes attention to the importance of chastity outside of marriage and to the form a truly Christian society might take, emphasizing that it would likely not correspond to modern political notions of right and left. Finally, Lewis emphasizes the dangers of pride, which is the sin from which all other sins flow.

The final section of the book consists of basic Christian theology, as Lewis understands it. Lewis discusses the idea of a three-personed God (the Holy Trinity) and of God as existing beyond linear human time. The bulk of his argument, however, concerns the ultimate purpose of Christian morality, which is to transform us into “sons of God” in the truest sense—that is, to enable us to partake not only in biological life but in the spiritual life of Christ. This process is difficult; in fact, it is a kind of death. By choosing it, however, we become a new sort of person—the sort of person God intended us to be—and more fully ourselves.