Over the course of the novel The Chrysalids, David has one goal: to keep his powers of telepathy secret from people who will find him guilty of blasphemy if his secret becomes "public" knowledge.
At one point in the novel, David meets Sophie (a girl with six toes on each foot). Both David and Sophie know that if her secret becomes known that she will be exiled by the community. Sophie's secret is found out and she is forced to flee. David, now possessing proof that no one can know about his powers of telepathy, tries even harder to conceal his secret.
David finds out that a society exists which will welcome him. HIs goal, therefore, is to reach Sealand without being found to be a telepathic. It is only David's strength to survive which allows him to make it to Sealand.
Therefore, David's strengths, which alow him to make it to Sealand, are: his faith in a better place, his ability to keep secrets, and his desire to be welcome as a telepathic.