Final answer:
Percy has learned the power of belief in oneself.
Step-by-step explanation:
The one important thing that Percy has learned so far in 'The Lightning Thief' is the power of belief in oneself. Throughout the story, Percy discovers that he is the son of a Greek god and is tasked with a dangerous quest. While facing numerous challenges and doubting his abilities, Percy learns to trust in his own strength and determination.
For example, when Percy confronts the minotaur, he initially doubts his ability to defeat the monster. However, as he taps into his own courage and embraces his identity as a demigod, he is able to overcome the challenge and save himself and his friends.
This theme of self-belief is reinforced throughout the novel, as Percy continues to face trials and learns to trust in his own abilities as a hero.