Not only did C.S. Lewis write the foreword to an English translation for Athanasius’ On the Incarnation, he has literally written on the topic of the incarnation throughout his writing.
He called it the grand miracle and it flowed through both his non-fiction and fiction work.
It’s how he thought of Aslan in terms of representing Jesus in Narnia. The Quotable Lewis records a letter Lewis wrote to a Mrs. Hook in December 1958:
If Aslan represented the immaterial Deity in the same way in which Giant Despair [a character in The Pilgrim’s Progress] represents despair, he would be an allegorical figure. In reality however he is an invention giving an imaginary answer to the question, ‘What might Christ become like, if there really were a world like Narnia and He chose to be incarnate and die and rise again in that world as He actually has done in ours?’ This is not allegory at all.
In “Christianity and Literature” from Christian Reflections, he wrote:
To believe in the Incarnation at all is to believe that every mode of human excellence is implicit in His [Jesus’] historical human character. . . . But if all had been developed, the limitations of a single human life would have been transcended and he would not have been a man; therefore all excellences save the spiritual remained in varying degrees implicit.
For Lewis, the incarnation was the pivot point of Christianity (and the entirety of human history). From his book Miracles:
The central miracle asserted by Christians is the Incarnation. They say that God became Man. Every other miracle prepares for this, or exhibits this, or results from this. Just as every natural event is the manifestation at a particular place and moment of Nature’s total character, so every particular Christian miracle manifests at a particular place and moment the character and significance of the Incarnation.
There is no question in Christianity of arbitrary interferences just scattered about. It relates not a series of disconnected raids on Nature but the various steps of a strategically coherent invasion—an invasion which intends complete conquest and “occupation.” The fitness, and therefore credibility, of the particular miracles depends on their relation to the Grand Miracle; all discussion of them in isolation from it is futile.
Christianity is a miracle religion. You cannot have Christianity without the miracles and, in particular, the incarnation. From “The Grand Miracle” in God in the Dock:
You could have a great prophet preaching his dogmas without bringing in any miracles; they are only in the nature of a digression, or illuminated capitals. But you cannot possibly do that with Christianity, because the Christian story is precisely the story of one grand miracle, the Christian assertion being that what is beyond all space and time, what is uncreated, eternal, came into nature, into human nature, descended into His own universe, and rose again, bringing nature up with Him. It is precisely one great miracle. If you take that away there is nothing specifically Christian left. There may be many admirable human things which Christianity shares with all other systems in the world, but there would be nothing specifically Christian.
Conversely, once you have accepted that, then you will see that all other well-established Christian miracles—because, of course, there are ill-established Christian miracles; there are Christian legends just as much as there are heathen legends, or modern journalistic legends—you will see that all the well-established Christian miracles are part of it, that they all either prepare for, or exhibit, or result from the Incarnation. Just as every natural event exhibits the total character of the natural universe at a particular point and space of time; so every miracle exhibits the character of the Incarnation.
One of the more shocking things about the incarnation is just how “unfair” it is. All of human history comes down to “a Jewish girl at her prayers.” From “The Grand Miracle” in God in the Dock again:
We, with our modern democratic and arithmetical presuppositions would so have liked and expected all men to start equal in their search for God. One has the picture of a great centripetal roads coming from all directions with well-disposed people, all meaning the same thing, and getting closer and closer together. How shockingly opposite to that is the Christian story!
One people picked out of the whole earth; that people purged and proved again and again. Some are lost in the desert before they reach Palestine; some stay in Babylon; some becoming indifferent. The whole thing narrows and narrows, until at last it comes down to a little point, small as the point of a spear – a Jewish girl at her prayers.
That is what the whole of human nature has harrowed down to before the Incarnation takes place. Very unlike what we expected, but, of course, not in the least unlike what seems, in general, as shown by nature, to be God’s way of working.
Walter Hooper, Lewis’ friend and editor, called Lewis “the most thoroughly converted man I ever met.” Christianity impacted every inch of who Lewis was and it was evident to those around him.
I believe it was the wonder, awe and, dare I say, magic of the incarnation that so resonated with Lewis. He caught the seminal importance of God becoming a man.
To Lewis, Immanuel was not something so easily shaken off. It was something that should shake all of who you are. God with us, the incarnation, changes everything. It did for Lewis.