In The Chrysalids by John Wyndham, David and the other discovered telepaths are escaping to and through the Fringes because they have been discovered and are in danger of torture and murder. Petra, because of her strong powers can communicate with the Sealand lady who promises to come and rescue Petra and David and the other telepaths escaping with them. David trusts Petra and eventually the Sealand lady comes within the range of his own telepathic powers and he can communicate with the Sealand lady himself. Because of these factors, Petra's communication with the Sealand lady, David's trust and belief in Petra and David's own communication with the lady, David believes in and is happy about the rescue operation coming for them from Sealand. After the rescue, when they are flying in to Sealand and when they are actually in Sealand, David feels it is not quite real, but the presence of his friends and sister assure him correctly that it is real and the buzz of a country full of telepaths gives all of them even greater assurance.
[Below you will find an eNotes Study Guide link and a reliable link to John Wyndham's biography and a link to a scholarly discussion of John Wyndham's early juvenile work.]