All records of miracles teach the same thing. In such stories the miracles excite fear and wonder (that is what the very word miracle implies) among the spectators, and are taken as evidence of supernatural power. If they were not known to be contrary to the laws of nature how could they suggest the presence of the supernatural? How could they be surprising unless they were even to be exceptions to the rules? And how can anything be seen to be an exception till the rules are known?
Lewis is here pushing back against the argument that because the ancients had so comparatively little understanding of how the universe works they were more predisposed to accept the miraculous. It's an interesting observation, at the same time accurate in its fashion--a miracle needs rules in order to break them--but also grossly negligent in its implications.
The truthfulness of the claimed miracle is never in question (certainly the Virgin Birth and Resurrection aren't), and so one could easily construe this to argue that a supposed miracle is true precisely because it is so contrary to accepted knowledge. For here a miracle is not a miracle in spite of its violation of our sense of the way the things work, but precisely because of it. The credulity this way of thinking cultivates, needless to say, is manna from Heaven to the world's charlatans and snake oil salesmen.