Lewis himself was, of course, an Anglican. Catholic admirers have often wondered why he didn't go "all the way" and become Catholic--indeed, there are no fewer than three books on the subject. (The most recent is actually by a Protestant, I believe.) But of course even asking the question presupposes a Catholic position. Tolkien was right, I think, in saying that Lewis became Anglican primarily because that was his heritage (and a very honorable reason that is, in my opinion). But certainly the relative lack of dogmatism of Anglicanism and its emphasis on the ancient consensus of the Church suited Lewis' temperament very well.
While Lewis did occasionally make criticisms of Catholicism or try to explain to Catholic friends why he saw no need to "convert," Lewis himself did not actively set "mere Christianity" over against "Roman" Catholicism, or indeed any particular Christian tradition. In Mere Christianity, he compared what he was articulating to the vestibule of a house. The goal, according to Lewis, was to choose a room and settle in.
But there's something intrinsically unstable about this metaphor. After all, the point of "mere Christianity" is that it covers basic truths all Christians agree on such as the Incarnation, the need for divine grace, the sacraments (at least baptism and Eucharist), the authority of Scripture, and so on. For any particular version of Christianity, these things are at the center of the "house"--they aren't simply the vestibule.
So it's not surprising that many people who adopt Lewis' slogan treat "mere Christianity" as a rival to more particular Christian traditions, such as Catholicism. Lewis has, in fact, given substance and focus to the longstanding Protestant rhetoric of distinguishing between "essentials" and "nonessentials." The fatal flaw in this approach has always been the disagreement among Protestants as to which items are "essential" and which are "nonessential." Lewis seems to point the way to a solution.
But in fact, the problem remains. For instance, a few years ago there was talk of starting a new "Great Books" Christian college, named after C. S. Lewis, which would have "mere Christianity" as its only standard. This was explained to me as being defined by the Nicene Creed. S. M. Hutchens, however, a writer for Touchstone (which bills itself as a journal of "Mere Christianity"), suggested that if the college wanted to be faithful to Lewis' vision of mere Christianity it should exclude anyone who believed in women's ordination, which Hutchens (claiming Lewis' support) believes is fundamentally incompatible with orthodox Christianity. (Hutchens made this statement in a comment which I now have trouble finding, but his view can be clearly seen in this later article about the conflict over women's ordination in the ACNA.)
The question of gay unions and "non-traditional" sexualities generally is an even more acid test of "mere Christianity" as a measure of orthodoxy. Many who are willing to accept disagreement on women's ordination, or even regard the ordination of women as the obvious move, see a "liberal" view on homosexuality or "gender identity" as incompatible with "mere Christian" orthodoxy. Yet there are many who accept historic creedal orthodoxy while believing that this is a point where new understanding of human psychology and biology needs to prompt a shift in how we think about sexuality, and that this is entirely compatible with the basic principles of traditional Christian sexual morality. So we have, at least, a three-way division among would-be proponents of "mere Christianity": those who accept both gay unions and women's ordination, those who accept women's ordination but not gay unions, and those who accept neither. Factor in the distinction between holding to one position or the other and thinking that it's an essential part of "mere Christianity," and things get more complicated. (I, for instance, am not convinced that "mere Christianity" is incompatible with any of these positions, although my own theological judgment is "liberal" on women's ordination and "conservative" on sexuality.)
"Mere Christianity" is by definition something that we can only see in the rear-view mirror. It provides little help in solving live debates in the present. Whenever the status of any one belief or practice is challenged, "mere Christianity" itself cannot solve the dispute--a living, concrete community is needed for that. But this, of course, takes us out of "mere Christianity" and back to the sectarian reality from which "mere Christianity" is supposed to deliver us.
So if Lewis was wrong that "mere Christianity" is merely the vestibule (because it's obviously the center of all the various forms of Christianity), and if "mere Christianity" can't function as an alternative to more concrete communities, where does that leave us? We need a concrete, rooted Christian tradition centered on "mere Christianity" but capable of addressing "borderline" issues and responding dynamically to the changing world.
I am not going to argue here why I think Catholicism fits that description better than the alternatives. Rather, the point I want to make is that for me Catholicism is not a rival to mere Christianity, so much as its fulfillment. As a Catholic, my faith is rooted deeply in precisely those elements of the faith that Lewis expounded so well--the ancient Creeds, the seven virtues, and the sacraments of Baptism and Eucharist.
The sacraments are a good place to see the relationship between "mere Christianity" and Catholicism. For Protestants, and thus for "mere Christianity" (since "mere Christianity" is by definition what Catholics, Orthodox, and Protestants have historically shared), there are two fundamental sacraments. For CAtholics, of course, there are seven. Yet Catholic teaching affirms that baptism and Eucharist are central to Christian life, and that the other five sacraments flow from them and are ordered to them.
To be Catholic, then, should not mean sneering at the concept of "mere Christianity." Yes, it is a poor substitute for Catholicism. But a Catholicism that does not have a clear sense of the "hierarchy of truths," that treats all Catholic teaching as just "stuff the Church tells us to believe," is a deeply incoherent and unconvincing Catholicism. If there is one common denominator in the many ex-Catholics I've talked to, it is, overwhelmingly, a failure to see the underlying structure of Catholicism. Instead, ex-Catholics describe Catholicism over and over again as a set of "rules" that had come to seem largely arbitrary to them.
Thus, Catholics need "mere Christianity" just as "mere Christianity" needs Catholicism. "Mere Christianity" is neither a vestibule leading to a variety of equally legitimate "living quarters," nor a coherent alternative to Catholicism and other concrete embodiments of Christian faith and practice. It is the beating heart of the Faith, the "hearth" shared by all members of our squabbling, tragic, confused family.
Without "mere Christianity," I would not have become Catholic. Indeed, I would not be a Christian at all. But without Catholicism, "mere Christianity" would remain an abstraction.
Lewis' most powerful and compelling account of "mere Christianity" is not in the book of that name, but in his essay "On Reading Old Books," appropriately prefaced to a translation of Athanasius' On the Incarnation by Sister Penelope Lawson. Here, Lewis presents "mere Christianity" as a reading strategy--a standard that gives us balance and sanity as we navigate the often confusing labyrinth of contemporary religious opinion. And this approach is rooted in his own experience (which he would describe at more length in Surprised by Joy).
I myself was first led into reading the Christian classics, almost accidentally, as a result of my English studies. Some, such as Hooker, Herbert, Traherne, Taylor and Bunyan, I read because they are themselves great English writers; others, such as Boethius, St. Augustine, Thomas Aquinas and Dante, because they were "influences." George Macdonald I had found for myself at the age of sixteen and never wavered in my allegiance, though I tried for a long time to ignore his Christianity. They are, you will note, a mixed bag, representative of many Churches, climates and ages. And that brings me to yet another reason for reading them. The divisions of Christendom are undeniable and are by some of these writers most fiercely expressed. But if any man is tempted to think—as one might be tempted who read only contemporaries—that "Christianity" is a word of so many meanings that it means nothing at all, he can learn beyond all doubt, by stepping out of his own century, that this is not so. Measured against the ages "mere Christianity" turns out to be no insipid interdenominational transparency, but something positive, self-consistent, and inexhaustible. I know it, indeed, to my cost. In the days when I still hated Christianity, I learned to recognise, like some all too familiar smell, that almost unvarying something which met me, now in Puritan Bunyan, now in Anglican Hooker, now in Thomist Dante. It was there (honeyed and floral) in Francois de Sales; it was there (grave and homely) in Spenser and Walton; it was there (grim but manful) in Pascal and Johnson; there again, with a mild, frightening, Paradisial flavour, in Vaughan and Boehme and Traherne. In the urban sobriety of the eighteenth century one was not safe—Law and Butler were two lions in the path. The supposed "Paganism" of the Elizabethans could not keep it out; it lay in wait where a man might have supposed himself safest, in the very centre of The Faerie Queene and the Arcadia. It was, of course, varied; and yet—after all—so unmistakably the same; recognisable, not to be evaded, the odour which is death to us until we allow it to become life:
an air that kills From yon far country blows.
We are all rightly distressed, and ashamed also, at the divisions of Christendom. But those who have always lived within the Christian fold may be too easily dispirited by them. They are bad, but such people do not know what it looks like from without. Seen from there, what is left intact despite all the divisions, still appears (as it truly is) an immensely formidable unity. I know, for I saw it; and well our enemies know it. That unity any of us can find by going out of his own age. It is not enough, but it is more than you had thought till then. Once you are well soaked in it, if you then venture to speak, you will have an amusing experience. You will be thought a Papist when you are actually reproducing Bunyan, a Pantheist when you are quoting Aquinas, and so forth. For you have now got on to the great level viaduct which crosses the ages and which looks so high from the valleys, so low from the mountains, so narrow compared with the swamps, and so broad compared with the sheep-tracks.
My own experience matches Lewis', except that I approached the Christian tradition from a radical Protestant perspective rather than from that of an atheist ex-Anglican. Like Lewis, I was struck by what all these authors have in common. But once my feet were on the "great level viaduct," I discovered that it was taking me somewhere. And, like the pilgrim John in Lewis' The Pilgrim's Regress, I found that it led me to Mother Kirk, in a somewhat more alarming and concrete way than Lewis himself intended by the term.