Lewis, on why it only makes sense that there be many ancient myths that sound similar to the Christian story
From God in the Dock
"To me, who first approached Christianity from a delighted interest in, and reverence for, the best pagan imagination, who loved Balder before Christ and Plato before St. Augustine, the anthropological argument against Christianity has never been formidable. On the contrary, I could not believe Christianity if I were forced to say that there were a thousand religions in the world of which 999 were pure nonsense and the thousandth (fortunately) true. My conversion, very largely, depended on recognizing Christianity as the completion, the actualization, the entelechy, of something that had never been wholly absent from the mind of man. (p. 132)"
"The heart of Christianity is a myth which is also a fact. The old myth of the Dying God, without ceasing to be myth, comes down from the heaven of legend and imagination to the earth of history. It happens — at a particular date, in a particular place, followed by definable historical consequences. We pass from a Balder or an Osiris, dying nobody knows when or where, to a historical Person crucified (it is all in order) under Pontius Pilate. By becoming fact it does not cease to be myth: that is the miracle. (God in the Dock 66)"