The Godless Boys by Naomi Wood

Imagine an alternative England, where the Church controls the country and non-believers have been exiled to a remote island. On the Island, a fierce group of boys patrols the community, searching for signs of faith and punishing any believers. When a new girl appears, arriving from the mainland to search for her long-lost mother, the gang is split: one boy falls in love with her, another seeks violent revenge. The struggle between them will change everything.