At the beginning of John Wyndham's The Chrysalids, I would suggest that one of David's most outstanding characteristics is his innocence and the trusting nature that comes with it.
His innocence is easy to see at the start of the story. When David meets Sophie, and accident while playing exposes her foot with six toes. David thinks nothing of it, though Sophie is greatly distressed that he has seen it, and Sophie's mom is also extremely upset, telling David Sophie will suffer if it is discovered, and begging him for his oath never to tell anyone what he has seen.
"It's very, very important," she insisted. "How can I explain to you?...If anyone were to find out they'd—they'd be terribly unkind to her."
David quickly promises to remain silent. At first, however, he cannot understand how having six toes could be such a big deal:
It was so heavy a promise that I was quite resolved to keep it...Though, underneath, I was puzzled by its evident importance. It seemed a very small toe to cause such a degree of anxiety.
David is still very uninformed. On his way home, he tries to understand. Why would Sophie and her mother be so fearful? In a moment of insight, things fall into place. David can begin to understand what society expects:
Then [the monotonous Sunday precepts join up] with a click that was almost audible..."and each leg shall be jointed twice and have one foot, and each foot five toes, and each toe shall end with a flat nail...And any creature that shall seem to be human, but is not formed thus is not human...It is a blasphemy against the true Image of God, and hateful in the sight of God."
One incident that foreshadows trouble is when Alan sees the imprint of Sophie's wet foot on the rock that she has just left. Alan is much too interested to simply be curious, and David feels a tangible threat:
I looked up and saw that [Alan] was staring at something beside me. I turned quickly. On the flat rock was a footprint, still undried...The mark was still damp enough to show the print of all [Sophie's] six toes clearly. I kicked over the jar...but I knew...that the harm had been done...he had turned and was standing looking along the bank towards the point where Sophie had disappeared into the bushes.
I ran up the stone and flung myself on him.
When David returns home, the news reaches his father that he has consorted with a mutant and kept her secret. To get information, David's father beats him badly. Perhaps it is his father's fury that finally opens his eyes to the dangers of the society in which he lives—something he had never understood before. Betraying his promise to Sophie is devastating.
By now it was not so much the bodily hurts that brought [tears]: it was bitterness, self-contempt, and abasement...'I couldn't help it, Sophie,' I sobbed.
What finalizes any doubts David might have is his Uncle Axel's explanation that he is in danger. David's telepathic abilities make him a mutant, and therefore, a threat. His uncle warns him:
I want you to keep it a secret. I want you to promise that you will never, never, tell anyone else what you have just told me—never. It's very important: later on you'll understand better how important it is. You mustn't do anything that would even let anyone guess about it. Will you promise me that?
David is a young man living in a society he does not understand. His father's ferocity, Sophie's capture, and his own abilities change the way he sees the world forever.