DJClayworth answered a question about the meaning of C.S. Lewis description of Christianity as a "true myth." What Lewis text uses the "true myth" terminology?
A quick Google search produced this:
Now the story of Christ is simply a true myth: a myth working on us the same way as the others, but with this tremendous difference that it really happened: and one must be content to accept it in the same way, remembering that it is God’s myth where the others are men’s myths: i.e., the Pagan stories are God expressing Himself through the minds of poets, using such images as He found there, while Christianity is God expressing Himself through what we call 'real things'. (goodreads quote)
More searching revealed that this is possibly from C.S. Lewis' Mere Christianity, but I was not able to find where. I am currently looking through the physical copy I have to find it and will update this answer once I do.
Edit: I found this letter, written by C.S. Lewis and addressed to Arthur Greeves on the subject of "the Myth of Christianity". The passage I quoted above is the first sentence of the third paragraph in the letter.