For one thing, in The Chrysalids, David's life is dominated by religion because the entire community of Waknuk is under religious rule: nothing occurs that is not first approved of and ruled upon by the religious leaders. The entire ideology of the community is built upon the precept of adherence to religious laws. Secondly, David's father is the religious Magistrate and as such is responsible for enforcing the religious laws. So from waking to sleeping, David's life is dominated by religion, religious laws and the enforcement of religion and its laws.
n addition, as the Magistrates son, David life is dominated by religion on a psychological level because he has seen cleansings of Blasphemies that remain unshakably in his thoughts. His life is dominated by religion on a social level because he has many friends, such as Sophie and Rosalind--and his sister Petra--who are Blasphemies because they have secret mutations and he chooses to protect them and their secrets. David's life is also dominated by religion because he keeps his own secret of mutational powers, a secret that would bring him before his father's unwavering, punishing, cleansing religious hand.