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.
If you're asking for the main theme, it'd probably be Deviation vs conformity.
- Intolerance of Deviation
The important issue of the intolerance towards Deviations is shown through the extreme reaction of Joseph to David’s comment.In the Chrysalids, the theme of Deviation is a constant theme that runs throughout the novel. In fact, it is the main focus of the book.Attitudes towards them, hence, is an important factor in deciding how the reader will view them and will also impact their choices on their future courses of action- if they had been accepted by society, the telepaths would not have had to run away from society and run such a risk to their lives. This attitude consists of prejudice, open hatred, ostracism and even killing intent.This can be seen from Joseph’s furious outburst when he heard what David heard. Such is the extent of his set attitude against Deviation that he employed selective hearing and chose not to hear David apologising and saying it was not meant to be said that way.Joseph represents the entirety of Waknuk Society’s view on Deviation; being the son of the one who pioneered them in the first place. Hence just the utterance of such Blasphemy – “If only I had another hand”- was enough to turn on his paranoia and make him display his utter intolerance of Deviation.He yelled at David, made everyone pray for ‘forgiveness’, and even implied he would punish David later, physically. It shows how Deviations are not tolerated at all in Waknuk society.
- Prejudice towards Deviation
The people of Waknuk believe that Deviants are an abomination and the work of the Devil. From a young age, the Definition of Man and the importance of Purity are drilled into them. This causes them to have a fixed mindset to persecute Deviants when they grow up. It can also be seen from how they regularly undergo inspections to destroy all Deviations from their property, like in the case of the Strorm household, especially Joseph, who take it as a personal insult to have a deviated crop or livestock in their farm. Deviants [Blasphemies] are cruelly sterilized and abandoned in the Fringes, never allowed to return. This harsh treatment of Deviations shows us that the theme of prejudice towards them is prevalent in Waknuk society.
- Importance of Conformity
Through Joseph Strorm’s harsh treatment of David, we can see how important the issue of conformity is to the inhabitants of Waknuk.Conformity in Waknuk is manifested in several ways, be it burning of Deviant crops, slaughtering of Deviant livestock and spreading awareness on the dangers of the Mutant to their peaceful society.Joseph shows this need for conformity in this passage by blasting David for even suggesting deviation from the Norm. The evidence for this is “you Blasphemed, boy. You found fault with the Norm,” “This is a terrible thing, an outrageous thing. You are…committing blasphemy!” Joseph immediately accused poor David of having blasphemed and started yelling at him, without mercy or bothering to understand the truth.His treatment of David therefore shows how important conformity to the Norm really is to him.
Of course, every other theme in the book is significant as well. These include the theme of fear of authority, oppression, punishment, Purity and the Definition of Man, Friendhip, Love, Sin, Telecommunication, Sacrifice, Death, Religion, Extremism, Superstition and so on. They all play an important part in carrying the plot forward.