Till We Have Faces: A Myth Retold is the last novel C.S. Lewis wrote, and the subject matter is something he thought about for more than three decades before actually writing the story. This novel is a kind of re-telling of the mythological story of Cupid and Psyche; it is written as a kind of an allegory of faith, primarily as seen through the character of Orual.
To the beautiful Psyche, Cupid's palace is glorious, but to her half-sister Orual it does not exist except for a moment of illusion, a misty kind of dream. This is the crux of Orual's internal faith struggle as well the motivations for her subsequent actions: does the god's palace (and, allegorically, God) exist.
Psyche has been given to the God of the Mountain as an appeasement, and she is happy in her marriage but has never seen her husband. In fact, that is the one thing she is not allowed to do. Orual, because she does not believe Psyche's insistence that her new home is beautiful, convinces Psyche to look at her husband's face, arguing that he is probably a monster (in fact, of course, he is the stunningly beautiful immortal god Cupid).
After Psyche makes this discovery, allegorically God has no choice but to banish her, a punishment for which, of course, Orual blames God rather than her own jealousy and envy. She spends many years trying to assuage her guilt, through deeds and the quest for knowledge, but it does not work.
The second part of the book is a new story, written by an older Orual who comes to realize that she was in the wrong, not God. It is clear that Orual wants to believe but does not, at least at first, "see" (understand) enough to know how to make her way to belief. Then comes her moment of allegorically accepting God by faith. In many ways, this is the story of Lewis's own journey of faith, which happened over time as he searched.
A second theme might be expressed with this Bible verse in I Corinthians 13:12:
For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
The entire concept of knowing and not knowing, hidden and revealed, having faces and not having faces is all connected to our (through the character of Orual) inability to see clearly here on earth. We can see, just like Orual's misty vision of Cupid's palace, but we cannot see enough--or perhaps just not well enough.
When the time comes to you at which you will be forced at last to utter the speech which has lain at the center of your soul for years, which you have, all that time, idiot-like, been saying over and over, you'll not talk about the joy of words. I saw well why the gods do not speak to us openly, nor let us answer. Till that word can be dug out of us, why should they hear the babble that we think we mean? How can they meet us face to face till we have faces?
In this novel, having faces means moving beyond the words we speak or the thoughts we think and seeing our real selves--jealousy, envy, and all. Of course this is often a journey that takes place over a long period of time; in Orual's case, a lifetime. She wanted to believe almost from the beginning but did not understand herself enough to do so.
Lewis does not really suggest here that God reveals himself to Orual; instead she is taken on a kind of journey of self-discovery by Psyche, and that is what she needed to do before she could find God. According to Lewis, then, it is as we face our own faults and flaws that we can meet Him face to face.