The Uses of Ignorance
Tolkien refers to this as Lewis's "Ulsterior motive," and sees it as key to his friend's religion in general and to the right reading of The Pilgrim's Regress in particular. We should take particular notice of the fact that Tolkien reads the Pilgrim's Regress in precisely the opposite way than CSL's early Catholic readers did: not as evidence for commitment to Catholicism, but as a savage repudiation of Catholicism.
Tolkien no doubt would say that those early readers were simply misled by the manifestly Catholic publisher of the book, along with that publisher's eagerness, as described by Mark, to encourage the assumption that the Pilgrim is a Catholic one. (I especially like a review that Mark didn't happen to quote, from The Living Church, an American Anglican journal: Norman Pittenger, an Episcopal priest with whom Lewis would butt heads later, wrote that the book's pilgrim ends "in a resting-place which we fancy is none other than the Church of Rome. Anglicans may wish that he had come their way, but Mr Lewis, who is a Roman Catholic, does not see it so.") For Tolkien these misunderstandings may readily be attributed to ignorance; he would claim a deeper insight based on years of intimacy with Lewis.
And yet Tolkien's interpretation is based on a faulty assumption: that Lewis had been thoroughly instructed from early childhood in a profoundly anti-Catholic disposition. Such a disposition would in fact have been seriously at odds with the genial and relatively high-church Anglicanism in which he was actually raised, in a house with Catholic servants, by the way—something that wouldn't have been tolerated by dyed-in-the-wool Ulster Prots, which Albert and Flora Lewis certainly were not. Lewis himself did not mean for his Regress to be read in that way, as he made abundantly clear over the years, for instance in a preface he wrote a decade after the book's first publication: "The book is concerned solely with Christianity as against unbelief. 'Denominational' questions do not come in." Presumably Tolkien knew this, but thought himself a better judge of such matters than the author: the derivation of "Ulsterior" from "ulterior," with its connotations of secrecy and hiddenness, indicate Tolkien's belief that Lewis knew not his own mind. (We need not dwell on that fact that Tolkien grew very annoyed indeed when readers interpreted his works with equal freedom.)
At this point we might recall a line I read earlier: "You will be thought a Papist when you are actually reproducing Bunyan"—a comment particular relevant to these interpretations of The Pilgrim's Regress, and especially to the book's invocation of "Mother Kirk." Catholic readers, accustomed to thinking and speaking of Holy Mother Church, focused on the "Mother," but had they thought more about the "Kirk" they might have recalled the Church of Scotland, the Scots Kirk, that Presbyterian body created in the image of the formidable Calvinist John Knox. One could say that the spirit of "mere Christianity" is at work in Lewis's decision to join "Mother" and "Kirk" in just this way; but readers see what they are prepared to see. And, it seems, a little, or even a lot, of knowledge is a dangerous thing in these matters. What those early Catholic readers knew, or thought they knew, and what Tolkien knew, or thought he knew, was precisely what led them astray.
These are very different readings of Lewis, but they have in common the belief that he can be placed, even fixed in a field of religious and cultural possibilities.
By contrast, consider a 19-year-old American, from a working-class family in Birmingham, Alabama, walking into a church bookstore in 1978, for the first time in my life, and seeing a paperback copy of Mere Christianity on the shelf. What does it tell me about its author? "C. S. Lewis was a professor of Medieval and Renaissance Literature at Cambridge University. His previous bestselling Macmillan books include … ." At that time I suppose I knew that Cambridge was an ancient and famous university; also that it was in England. That may have been the extent of my knowledge, except that I had a vague idea that Lewis was a defender of Christianity and an author of children's books. Other early American editions of Lewis's books offered scarcely more information: most typically they identify him simply as "Fellow of Magdalen College, Oxford"—this note persisted in editions printed long after his relocation to Cambridge and even after his death—and then list a few of his books.