In his "Preface to Paradise Lost", Lewis wrote the following:
"The first qualification for judging any piece of workmanship from a corkscrew to a cathedral is to know *what* it is - what it was intended to do and how it was meant to be used. After that has been discovered the temperance reformer may decide that the corkscrew was made for a bad purpose, and the communist may think the same about the cathedral. But such questions come later. The first thing is to understand the object before you: as long as you think the corkscrew was meant for opening tins or the cathedral for entertaining tourists you can say nothing to purpose about them."
This is a splendid piece of writing, but the idea presented is no way an original one - Plato and Aristotle said the same, said it clearly, and said it over two thousand years before Lewis did. If you had been able to confront Lewis with this fact, he might have said "Exactly."
This brings us to one of the great themes of Lewis's writing, evident nowhere more so than in "Mere Christianity": the defense of traditional wisdom against prejudice of our age that would reject it for no other reason than that it is traditional. Lewis often encountered those who complained that his ideas were old-fashioned, and his standard reply was that theirs would soon be as well, so in that they were equal. I admit I couldn't help but smile at the complaint by one Amazon reviewer that Lewis's ideas on sexuality were "decades old". The complaint is quite mistaken: the ideas are not decades old but thousands of years old.
And it is here that we have part of the answer to the problem of understanding the kind of thing "Mere Christianity" is: it is nothing new. It is in fact very, very old. What Lewis is defending is not his own personal belief system, but the Christianity that is the common heritage of mankind. The threat to it comes not from hard-core atheists, who receive the barest of notices from Lewis, but the general modern tendency to subject traditional Christianity to the death of a thousand cuts - discarding one ancient doctrine after another, on grounds often no better than mere chronological snobbery - that modern people aren't supposed to believe that kind of thing anymore
This is why Lewis, in what has been often described as the most important defense of Christianity in the twentieth century, spends a mere fifteen pages in arguing for the existence of God. The important task is not to defend a vague theism, which is the position Lewis found from experience that his audience already believed, but to rebuild what little of traditional Christianity modernism has left them - some vague belief in "a higher power", and "some purpose to all of this" into that concrete set of specific beliefs that are the historical core of Christianity.
While the defense of historical Christianity is one part of understanding what "Mere Christianity" is, the other part is that it is intended to be accessible to anyone. This requires that Lewis be both clear and brief - a combination brutally difficult to achieve, as any writer who has attempted it will attest.
Lewis's success in this can be measured in two ways: first, that his work has indeed found a very wide readership - millions of have read it; second, his work is often recommended by those whose knowledge of traditional Christian theology is broad and deep. The size of the readership attests to the accessibility of the work, and the expert recommendations attest to the accuracy of its message.
There is one other thing that is important to note about Lewis's success: Lewis could afford to be brief because what he was explaining was not his own theology, but our common intellectual inheritance. The reader who is dissatisfied with the depth of this or that point in "Mere Christianity" will have no difficulty in finding sources that go into the same thing in much greater detail. Calvin wrote line-by-line commentaries on all of scripture. Thomas Aquinas's "Summa Theologica" is over 6,000 pages long. The collected works of Augustine fill more than 40 volumes.
So, to return to the question with which this review began: what kind of thing is "Mere Christianity"? The answer is that it is a brief exposition of traditional Christianity for a modern audience. In the sixty years since it was published, the nature of the modernist challenge to Christianity has not substantially changed, nor has a clearer, more accessible response to that challenge yet been written. Some have complained that the work has "gaps" or that it skims over this or that point, but that is a complaint that fails to understand what kind of thing this is. What they are asking for, whether they know it or not, is a completely different book. Properly evaluated, on the basis of the kind of thing it is, it is trivially easy to give the highest recommendation to "Mere Christianity": it is on a topic of the greatest possible importance and the presentation is outstanding.