Back when he was an atheist, C.S. Lewis wrote to a friend, "All religions, that is, all mythologies to give them their proper name, are merely man’s own invention – Christ as much as Loki." So after he became a theist, when he began to consider Christianity, Lewis had to deal with the question of how much of it was myth and how much was fact. We pick up (from Part 1) his discussion from chapter 14, "The Grand Miracle," from his book, Miracles; a Preliminary Study:
From this point of view the Christian doctrine makes itself so quickly at home amid the deepest appehensions of reality which we have from other sources, that doubt may spring up in a new direction. Is it not fitting in too well? So well that it must have come into men's minds from seeing this pattern elsewhere, particularly in the annual death and resurrection of the corn? For there have, of course, been many religions in which that annual drama (so important for the life of the tribe) was almost admittedly the central theme, and the deity - Adonis, Osiris, or another - almost undisguisedly a personification of the corn, a "corn-king" who died and rose again each year. Is not Christ simply another corn-king?
Now this brings us to the oddest thing about Christianity. In a sense the view which I have just described is actually true. From a certain point of view Christ is 'the same sort of thing' as Adonis or Osiris (always, of course, waiving the fact that they lived nobody knows where or when, while He was executed by a Roman magistrate we know in a year which can be roughly dated). And that is just the puzzle. If Christianity is a religion of that kind why is the analogy of the seed falling into the ground so seldom mentioned (twice only if I mistake not) in the New Testament? Corn-religions are popular and respectable: if that is what the first Christian teachers were putting across, what motive could they have for concealing the fact? The impression they make is that of men who simply don't know how close they are to the corn-religions: men who simply overlook the rich sources of relevant imagery and association which they must have been on the verge of tapping at every moment. If you say they suppressed it because they were Jews, that only raises the puzzle in a new form. Why should the only religion of a 'dying God' which has actually survived and risen to unexampled spiritual heights occur precisely among those people to whom, and to whom almost alone, the whole circle of ides that belong to the 'dying God' was foreign? I myself, who first seriously read the New Testament when I was, imaginatively and poetically, all agog for the Death and Rebirth pattern and anxious to meet a corn-king, was chilled and puzzled by the almost total absence of such ideas in the Christian documents. One moment particularly stood out. A 'dying God' - the only dying God who might possibly be historical - holds bread, that is, corn, in His hand and says, 'This is my body.' Surely here, even if nowhere else - or surely if not here, at least in the earliest comments on this passage and through all later devotional usage in ever swelling volume - the truth must come out; the connection between this and the annual drama of the crops must be made. But it is not. It is there for me. There is no sign that it ws there for the disciples or (humanly speaking) for Christ Himself. It is almost as if He didn't realise what He had said.
The records, in fact, show us a Person who enacts the part of the Dying God, but whose thoughts and words remain outside the circle of religious ideas to which the Dying God belongs. The very thing which the Nature-religions are all about seems to have really happened once, but it happened in a circle where no trace of Nature-religion was present. It is as if you met the sea-serpent and found that it disbelieved in sea-serpents: as if history recorded a man who had done all the things attributed to Sir Launcelot but who had himself never apparently heard of chivalry.