The people of the Fringes are treated badly in David's society, considered enemies to the people because they are mutated and raid the community from time to time. David begins to question the beliefs and actions of his community when he meets Sophie and because he can communicate with others through his mind.
The Fringe people are mutants who have been banished from normal society because of their mutations. They subsist on small areas of land that clearly cannot support them. So they raid the community, sometimes to just take food and livestock, and sometimes in large groups, to cause havoc and destruction. So they are considered to be enemies, to be banned or even killed. They are held to be particularly worrisome because many of them appear to be normal, with no mutations that are obvious to the eye. The hatred for mutation is the basis for David's society, which seeks to rid mankind of all mutation. They have elevated this into an entire religion, the belief of which is that any mutation is a blasphemy in God's eyes. It must be rooted out, through death, in many cases. This holds true not only for people, but also for plants and animals. All mutations must be eliminated. This is an obsession, and no one in the community is more obsessed than David's father.
David has two reasons to question this "gospel." He meets Sophie, who is a lovely girl, and accidentally discovers when she injures her ankle and he helps her by taking off her shoe, that she has six toes on her foot. This is a mutation, but he cannot see how Sophie can be an abomination to God. He keeps her secret to protect her from his father and others who would seek to harm her, and he has a difficult time reconciling the ideas of the society with his newfound friend's mutation. He also is able to communicate through extra-sensory perception with his half-cousin Rosalind and a few others in the area. When his Uncle Axel discovers this, he warns David to never reveal this to anyone else. David understands that this would put him and the others in danger, a difference and likely a mutation that the community would find appalling and want to be rid of. David's father, being the zealot that he is, would not hesitate to root out the blasphemous mutation of his son. Thus, David's friendship with a mutant and his own mutation have made him reconsider the society's entire basis.