Two themes are no good comes from an oppressive society and stand up for your friends.
The first theme in the story is that oppressive societies eventually implode. While we don’t see that happen yet, we see the beginnings of it. Waknuk’s society is based off of religious principles supposedly derived from some post-apocalyptic event. People feel that anyone or anything that does not conform to the True Image is blasphemous and you should get rid of it. That includes anything from crops to people.
When David and the other telepaths meet the Sealand woman (in their minds at least) she explains to them that the way they have been treated is wrong, and the sign of a backwards society.
The static, the enemy of change, is the enemy of life, and therefore our implacable enemy. [Consider] some of the things that these people, who have taught you to think of them as your fellows, have done. [The] pattern scarcely varies wherever a pocket of the older species is trying to preserve itself. (Ch. 17)
Although the Sealanders may be advanced enough to accept change, that does not help the people of Waknuk. They live under a state of religious tyranny. When people are trying to hide babies and committing suicide, you know that there is trouble in your community.
When David is still a little boy, he meets Sophie and learns that she has six toes. This makes her blasphemous in Waknuk, and her parents should have turned her in when she was born. Children need a certificate that shows they meet the requirements of the True Image. If they don’t get it they are cast out. Sophie’s parents did not turn her in, and instead hid her.
David tries to keep Sophie’s secret, but one day another boy sees her and David with her. His father finds out and asks him about it. David tries to keep from telling him anything, trying to protect Sophie as long as he can. His father beats it out of him though, and he feels terrible.
[My] tears soaked into my pillow. By now it was not so much the bodily hurts that brought them: it was bitterness, self-contempt, and abasement. (Ch. 5)
David tried to protect Sophie. He also tries to protect the other telepaths when they have to go on the run. He believes in the importance of looking out for your friends, no matter how dangerous it is.