Answer: Theme
Step-by-step explanation:
The simply refers to the main idea that the writer wants to explore in a story, novel, or other literary work. It should be noted that the theme of a particular story can be conveyed through the use of setting, plot or dialogue.
Therefore, "This book is a warning to the living that atrocities like the Holocaust can happen and will happen again if we are not vigilant" is a theme as it expresses the idea of the writer.