In Chapter 4 David is discovered by Uncle Axel chatting away to his other telepath friends. David tells his Uncle Axel his secret - that he is able to communicate in his mind with others like him - because Uncle Axel is his "best friend among the grown ups." This is a key moment for both David and the rest of his companions, as Uncle Axel makes him keep this secret and they all agree that this is a very wise move:
His gravity impressed me greatly. I had never known him to speak with so much intensity. It made me aware, when I gave my promise, that I was vowing something more important than I could understand.
Having shared his uncle's seriousness and gravity with the others later on that night, they are all relieved to make this joint promise. As David says, this promise also had the effect of forming them into a group because the promise signified the "formal admission of our responsibilities towards one another." From this point on in the novel, the group are engaged in a quest for self-preservation that involves them working together.
Axel found out that his nephew David is a "deviation", and that their are others like him , including Petra, David's little sister.