Answer:
Reading the entire text
Step-by-step explanation:
By reading the entire text, one can grab and understand the story's plot, its whole characterization, and the main conflict of the story. Hence, even if the characters and subplots change through the story, one will still be able to identify the actual theme of the story.
Therefore, the prerequisite to understanding the major or recurrent theme discussed in a text is "by reading the entire text."