i) C. S. Lewis is probably best-known and certainly best-loved for his Christian allegories (e.g. The Chronicles of Narnia, Perelandra).
For some reason, Lewis eschewed the "allegorical" classification. His denial seems to operate with a specialized definition of "allegory."
But several books clearly allegorize Christian theology: crucifixion, Resurrection, and atonement (The Lion, the Witch and the Wardrobe), creation and fall (The Magician's Nephew), final judgment and heaven (The Last Battle), alternate history of the Fall (Perelandra).
Aslan represents Christ while Lucy represents the theological virtue of faith (and hope).
ii) There is, however, a sense in which The Chronicles of Narnia are a double allegory.
The Lion, the Witch, and the Wardrobe opens with four young refugees during the the Blitz. And in reality, Lewis hosted four girls during WWII–Margaret, Mary and Katherine, and June Flewett.
iii) The character of Lucy was named after Lucy Barfield. However, the character was modeled on June (stage name Jill) Flewett–who was a pious Catholic schoolgirl:
So the character of Lucy is based on a real person whom Lewis knew. In fact, I wouldn't be surprised if June wasn't the daughter that Lewis never had. She may well have triggered his latent paternal instinct.
And that makes sense. Lucy is clearly Lewis's favorite character. And she's the most real character. Individualized in a way that the other Pevensie siblings are not. Lewis may have known girls like Susan, but with a certain diffidence. Peter and Edmund merely exist as dramatic vehicles to personify theological lessons. They don't have much personality.
But if Lucy allegorizes June Flewett, then Prof. Kirke allegorizes C. S. Lewis–while Kirke's residence allegorizes the Kilns. Likewise, the backyard inspired the Narnian landscape:
The extensive wild grounds to the south of the house(comprising a lake and a wooded hill) provided the inspiration for the Chronicles of Narnia.
iv) Critics think there's a thick layer of oral tradition, redaction, and legendary embellishment separating the Gospels from the historical Jesus: the "Jesus of history" from the "Christ of faith."
Of course, I disagree. However, let's play along with that claim for the sake of argument.
In many respects, The Chronicles of Narnia are far removed from reality. Not only are they fictional, but belong to the sword-and-sorcery fantasy genre. Full of magic. Populated by talking animals, fauns, centaurs, &c.
And yet, as we've just seen, in key respects The Chronicles of Narnia are grounded in reality. A paper-thin allegory of some real people, real places, and real events.
If The Chronicles of Narnia can be that historical just under the surface, despite the imaginary details and mythopoetic overlay, then in principle, the Gospels could be quite historical even though, from a critic's perspective, they appear to be legendary.