The main theme of The Chrysalids is that we should embrace change instead of fearing it.
There are many themes in the book, but the idea that change is not dangerous is an important one. When the Sealand woman arrives to save the telepaths, she comments that David’s people are the ones who are going to be extinct, because they do not allow change.
They have become history without being aware of it. They are determined still that there is a final form to defend: soon they will attain the stability they strive for, in the only form it is granted -- a place among the fossils. . . .' (ch 16)
The people of Waknuk have lived in fear for two generations, even since David’s grandfather founded the place. They have interpreted the Bible specifically enough that they are convinced that they know exactly what a person should look like.
And God decreed that man should have one body, one head, two arms and two legs: that each arm should be jointed in two places and end in one hand: that each hand should have four fingers and one thumb… (ch 1)
This limited view of life extends to their burning crops and plants that are deviant from what they expect. Therefore there is no change, and no evolution. The people are stuck where they have always been, and they refuse to allow change.
Joseph Strorm and his people are so desperate to keep everyone in line that he even beats his son until he gives up Sophie and then comes after him with an army when he and the other telepaths flee with David’s little sister to the Fringes.
There are no human emotions in the rule of law in Waknuk. Everything is ruled by fear and dogma. Innocents are sacrificed so that people can have a feeling of control.