Now the story of Christ is simply a true myth: a myth working on us in the same way as the others, but with this tremendous difference that it really happened.
He was referring to mostly Greek mythology and how it had worked itself on people. He claims the story of Christ also works like a myth but with the awareness that it actually happened.
Lewis and Tolkien Debate Myths and Lies
https://www.youtube.com/watch?time_continue=39&v=NzBT39gx-TE
Worth a look.
Is the story of Christ a myth? Certainly it is, the most powerful myth ever. Why? Because it is true. Christ is the Son of God. All other truth, all other myth, crashes upon the shore of Christ as Savior of the world.