I would say that the the greatest truth to be learned from the diary of Anne Frank is the power of the human spirit. Anne, her family, and some of their friends, are forced to live hidden lives in a secret annex in Nazi-occupied Amsterdam. As Jews, they are in serious danger of being captured by the Germans at any moment and sent to their deaths in a concentration camp. Inevitably, this desperate situation generates enormous challenges for those forced to endure the hardships of life in such a claustrophobic environment.
Despite these circumstances, Anne is still able to retain a remarkable degree of humanity. Her spirit is so strong that she is able to lead a much more normal life than many people would in her situation. If anything, she becomes even stronger as the diary progresses; she achieves an admirable degree of maturity amidst the constant worry and the incessant prospect of discovery.