Why does david question the definition of 'blasphemy' in The Chrysalids?

David questions his community's definitions and ethics, including the definition of blasphemy, because his experience has shown him that there is a different way. Sophie is his first encounter with a deviant... and she is cool, and becomes his best friend. He sees how her family is different from his, and he wants that rather than what he has. Then he sees the community destroy that which he has learned to love. There are other examples... his uncle's views about his own difference, and that secrecy is best, his observation of his father's brother when he is captured and he sees him in Waknuk, and his observation of what happens to his aunt and her daughter.