Main ideas are usually attributed to short stories, since they are more concise and focus on dealing with one or two specific themes. Novels, especially one as lengthy and complex as Pride and Prejudice, compile the different stories of many characters, keeping some similarities that will become the central themes of the literary work. Themes manifest in motifs (repetitive issues), and symbols which reinforce their impact on the plot.
In Pride and Prejudice Jane Austen cleverly begins her novel with an introduction that reunites all of the central themes of the novel: "Love versus Class and Reputation".
It is a truth universally acknowledged, that a single man in possession of a good fortune, must be in want of a wife.
This, "universally acknowledged truth" means that Regency society had a habit to expect young men to inherit property from someone in their family and, by default, "coming to property" would mark social and financial success = Class.
That the single man in possession of a good fortune MUST be in want of a wife entails that all available women need to prepare themselves for the opportunity of having this fortunate man consider them as potential wives. This is why woman had to keep a very clean profile which, with the exception of the scandalous Lydia, had to remain intact= Reputation.
The fact that this match is made merely out of convenience is what brings out the central idea of TRUE LOVE out of the novel; our main character refuses to abide by the social custom of accepting a suitor merely for financial gain. The males equally seem hesitant to accept a potential wife merely out of being told to do so. Yet, it is in the fact that social class came in between that Pride and Prejudice bases its central theme of "Love, against Class, against Reputation"