Why didn't C.S. Lewis convert to Catholicism?

In his book Mere Christianity he indicates that Christianity is one step in a sort of religious evolutionary system. In his mind the thing after Christianity won't be constrained by religious divisions or dogmas. This is one obvious reason he wasn't interested in becoming a Catholic.

The evolution is approximately: Paganism → Christianity → Something Better.

Perhaps a modern man can understand the Christian idea best if he takes it in connection with Evolution. (p. 218)

(Never Forget that we are all still 'the early Christians'. The present wicked and wasteful divisions between us are, let us hope, a disease of infancy;...) (p. 221)

On this view the thing has happened: the new step has been taken and is being taken. Already the new men are dotted here and there all over the earth... Every now and then one meets them. Their very voices and faces are different from ours: stronger, quieter, happier, more radiant... They will not be very like the idea of 'religious people' which you have formed from your general reading... And I strongly suspect (But how should I know?) that they recognize one another immediately and infallibly, across every barrier of colour, sex, class, age, and even creeds. In that way, to become holy is rather like joining a secret society. To put it at the very lowest, it must be great fun." (p. 223)

Mere Christianity means we are all Christians if we believe the essential doctrines of the faith. A Protestant does not need to become Catholic any more then a Catholic needs to become a Protestant. The things we differ on are insignificant, if a Jesus Christ died on the cross for you then we are all good. If he did not then that is another discussion all together. C.S. Lewis was writing to all Christians: Protestant, Catholic, Russian Orthodox, Greek Orthodox, Eastern Orthodox and anyone else who believes He died for us and rose on the third day. Lewis had no need to become a Catholic that is part of his infinite value, we all get to benefit from him, and that is a good thing.

But in general, we like him, so why didn't he like us?

In addition to the quotes from the obvious books to consult C. S. Lewis and the Catholic Church and C.S. Lewis and the Church of Rome, an National Catholic Register article in 2017 titled Why C.S. Lewis Never Became a Catholic cited several more references and interviews with Peter Kreeft, George Sayer (biographer and friend), Al Kresta (Catholic radio host), mentioning factors such as deep Belfast prejudice (see Ivan Sayer's answer as well), Mary, and Papalism as possible reasons.

Please don't get me wrong; I am a great admirer of C.S. Lewis, having read most of his books and credited him with many breakthrough for me to understand 1) how to integrate Christianity into my worldview, 2) chart a path for authentic Christian faith, as well as 3) paving a path for me to appreciate Thomism, St. Augustine (as synthesized by Aquinas with Aristotle), and medieval worldview (which has been so maligned unfairly in modern times esp. by Protestants). So I definitely understand why there are many Catholic authors, theologians, and lay people who came to find a kindred spirit in C.S. Lewis, and hence the question "why didn't he convert?"

I was not raised in Belfast, but having been raised Calvinist since birth I find it hard to adopt certain Catholic practices regarding Mary and the Pope, even though I respect and agree with a lot of the teaching of Pope Leo XIII, Pope JP II, and Pope Benedict XVI as well as the doctrine of Purgatory and real presence in the Eucharist. The closest I can get to Roman Catholic is to becoming a conservative High Church Anglican member. (But unlike C.S. Lewis I adore the hymns and the liturgy.)

So the explanation offered in the NC Register article above feels right and shouldn't taint C.S. Lewis's desire to promote common elements of "Mere Christianity" to his readership while backpedaling elements that cause schism. I appreciate C.S. Lewis to be authentic and true to himself, preferring to talk of 1) himself as a sinner before God, 2) how we can become better Christians by cooperating with the Holy Spirit to let our hearts to be transformed, and 3) clearing wrong reasons in our head, rather than emphasizing his denominational preference. I think that's why he has been a successful apologist for all Christians.

This answer is my personal opinion, no more, but it is the personal opinion of an aging man who has read almost every word Lewis ever wrote, most more than once.

When Lewis returned to the faith, he was doing what many do, looking for a spiritual home that was also a social orthodoxy. This can be seen if you read his memoirs, Surprised by Joy. Shortly after he reconverted to mere theism, and before he was a believing Christian, he began to attend church. This suggests that he was not merely searching for a credible metaphysic, but also a community in which it was believed. In middle class England, where he worked for most of his adult life, that orthodoxy had to be Anglicanism. Moreover, Lewis was brought up in the Anglican church of Ireland. He was returning to something he knew.

The Protestant Catholic distinction is no longer a heavily political distinction in England, but it once was. Remember, the battle of Marston Moor was fought between Royalists (overwhelmingly Catholic) and Parliamentarians (overwhelmingly Anglican Protestant). Remember, when Newmann and Manning migrated from Protestant to Catholic in the middle of the 19th century there were many disapproving comments and raised eyebrows.

My last paragraph is, on reflection, over-simplified — but it gets to one of the points I'm trying to make. If you read the Wikipedia article on the Elizabethan Religious Settlement it becomes very clear that the creation of a separate Christian denomination called the English Church or Anglicanism was a highly political business in which both houses of parliament were involved. We now talk as though the differences between Anglicans and Catholics were differences of metaphysical doctrine. This is not quite so; but it is what happens to the church in political squabbles. When the squabbles are resolved, subsequent generations are left with differences of doctrine which they argue about without paying attention to their political origin. This overlooking animates both the question we are trying to answer and Lewis' own remarks in Christian Reunion. Right down to Lewis' own youth, even though the Catholic Church was and is an essential part of the Religious Physiognomy of Britain, to be orthodox was to be Anglican. If you are brought up orthodox and involved in certain professions, you don't think about orthodoxy, you 'roll with it'.

I have often speculated that if Lewis had survived much longer, sound in mind, wind and limb, he would have considered, at least, becoming a Catholic. (If memory does not deceive, his mate Walter Hooper did migrate to Catholicism.) I doubt, however, whether he would have given much attention to the political side of his choice. Politics was not his favorite aspect of human life.

C. S. Lewis wrote in Christian Reunion:

The real reason, I take it, why you cannot be in communion with us is not your disagreement with this or that particular Protestant doctrine, so much as the absence of any real "Doctrine", in your sense of the word, at all. It is, you feel, like asking a man to say he agrees not with a speaker but with a debating society.

And the real reason why I cannot be in communion with you is not my disagreement with this or that Roman doctrine, but that to accept your Church means, not to accept a given body of doctrine, but to accept in advance any doctrine your Church hereafter produces. It is like being asked to agree not only to what a man has said but to what he's going to say.

To you the real vice of Protestantism is the formless drift which seems unable to retain the Catholic truths, which loses them one by one and ends in a "modernism" which cannot be classified as Christian by any tolerable stretch of the word. To us the terrible thing about Rome is the recklessness (as we hold) with which she has added to the depositum fidei - the tropical fertility, the proliferation, of credenda. You see in Protestantism the Faith dying out in a desert: we see in Rome the Faith smothered in a jungle.

I know no way of bridging this gulf.

That said, Lewis usually takes great pains to avoid sectarian division (perhaps influenced by growing up in Northern Ireland), and is often sympathetic to Catholic viewpoints. For example, he received advice on Mere Christianity from Catholics as well as Protestants before its publication, to ensure that it expressed sentiments that were universally agreeable.

The book C. S. Lewis and the Catholic Church by Joseph Pearce (Ignatius Press, 2003) explores the development of Lewis's thought on this matter. The foreword by Thomas Howard summarises as follows:

Lewis wishes us to accept his identity as a "mere" Christian; but we find the truth of the matter is that he is a mere Protestant. [...] But there are more anomalies. Although he loathed the whole business of "High", "Broad" and "Low" churchmanship in the Church of England, he could not avoid it all. He had nothing but contempt for the Broad churchmen who diluted the Faith until it was a mere sickly gruel (see his Anglican bishop in The Great Divorce). And he detested the "smells-and-bells", lace-cotta, biretta sort of thing that one finds at Anglo-Catholic shrines, and which formed the metier of T. S. Eliot. He just wanted to be left alone, to go to church and be done with it.

But it was not so simple. In spite of himself, Lewis moved more and more toward what can only be called a "catholic" Anglicanism. Again - he hated the epicene punctilio of the Anglo-Catholic party: but his faith came to embrace all sorts of doctrines and practices that his evangelical readers (who are his most enthusiastic clientele) must sedulously ignore. He spoke of "the Blessed Virgin", and made his confession to his priest regularly, and believed in purgatory, and even came to refer to the Eucharist as - heaven help us all - the Mass! Lewis's anti-Romanism remained with him until his death, however.