C.S. Lewis loved old books. In a short piece he wrote to introduce Athanasius’s On the Incarnation to a modern audience, he admonished that Christians who only read new books are joining “at eleven o’clock a conversation which began at eight” and “will often not see the real bearing of what is said.” He counseled reading old books to put “the controversies of the moment in their proper perspective.”

What exactly was he after in the old books? Steadiness. Surety. A clear foundation from which to build and critique that which appears novel. Lewis concluded, “The only safety is to have a standard of plain, central Christianity (“mere Christianity” as [Richard] Baxter called it).” While there are many fine reasons to commend the writing of C. S. Lewis for the modern Christian, perhaps Lewis’s admonishment now applies to his own work—for many would see a book published in 1952 as quite old.

Lewis wrote that phrase “mere Christianity” in 1944, eight years before the publication of his book by the same name. He wrote the book because, in part, during those war years, Lewis was invited to leave the comforts of his books at Oxford University to travel to London and endure potential German barrage to deliver radio addresses on the topic, “What Christians Believe.” Lewis was asked not because of his scholarly credentials, though he had them, but because he was an Anglican layman who converted to Christianity as an adult from atheism. The producers thought he would speak to the common listener.

Clyde Kilby, American preserver of Lewis’s legacy, also saw Lewis as an ally for the ordinary Christian, but not just because of Lewis’s ability to write with clarity. Lewis, Kilby said, is an ally to Christians because he is a Christian. His arguments and assertions of biblical truth do not belong to the world. Lewis “belongs to us.”

However, defining “us” is not easy these days. Tribal factions, debate over how Christians are to relate and try to transform the structures of society, and to what end, have left many defining evangelical Christianity more by what one doesn’t want it to be than the sturdy core to which Lewis calls. A return, then, to a nice, hot bowl of Mere Christianity might just be what the doctor ordered for what ails us.

Lewis did not write to define denominational boundaries. Rather, to a nation wondering whether it would survive a war, he “thought the best, perhaps the only, service I could do for my unbelieving neighbours was to explain and defend the belief that has been common to nearly all Christians at all times.”

To accomplish this, he explained the various expressions of Christianity as doors opening to rooms off a central hallway. His aim was to bring his readers out into the hall to identify the core of Christianity they all share under one roof.

But this is not to say that Mere Christianity did not point readers to the value of the confines of local churches and traditions. For it “is in the rooms, not in the hall, that there are fires and chairs and meals.”

Indeed, the reading of Mere Christianity reveals there is more to the mereness than one might at first think. In four sections, Lewis talked at length about the virtues of Christianity, the value of marriage, the relationship of Christianity to psychology, and an overview of the doctrine of the Trinity.

Further, the way Lewis wrote commends the book to be read and re-read. Not only did he present orthodox Christianity in simple terms, he also encased his presentation in memorable and lasting analogies.

For the former, Lewis famously posited his argument that Jesus Christ “was, and is, the Son of God; or else a madman or something worse.” For the latter, he used sheet music and piano keys to explain the existence of a moral law, on the one hand, and, on the other, compared God to a dentist who cures a single toothache of sin, yes, but also treats the root of the malady in full.

Mere Christianity is not a perfect book. J. R. R. Tolkien, who loved Lewis, did not agree fully with Lewis’s talk about marriage and divorce—and many have found other points of difference. However, the central claims remain worth reading, and many are still reading and re-reading Mere Christianity. Christians should read this book simply because it has been used to shape the lives of thousands.

To wit, as Lewis’s chapter on “The Great Sin” was instrumental to my own early Christian formation as a college student, I recently used social media to ask friends for their Mere Christianity thoughts and experiences. Everyone from my own daughter, to Lewis experts far and wide, to friends in the United Kingdom replied with helpful affirmations of the ongoing value and commendable virtues of Mere Christianity, many of which informed this article.

While Lewis would admonish Christians today to read old books to find the secure foundation of mere Christianity, he would also want them to read old books to propel them forward. In his chapter on hope he said, “If you read history you will find that the Christians who did the most for the present world were just those who thought the most of the next.”

To put the controversies of the moment in their proper perspective, every Christian should read this old book, at the very least, to grow in the knowledge and enjoyment of the timeless One to whom it directs their thoughts.

Editor’s Note: This post originally appeared at Lifeway Books.