The central theme of Madame Bovary is the desire of a woman educated by novels, who expects from life what exists only in books, and the bill that reality presents for that difference, a bill that arrives in money, and not in a sermon.
The book shows the mechanism through repetition. The language of romance that Emma exchanges with Léon at dinner on her arrival in Yonville returns, with another intention, in Rodolphe's seduction at the agricultural show. With each man the cycle is similar, exaltation and then boredom, until, with Léon in Rouen, she finds "in adultery all the insipidity of marriage." Writing to her lover, Emma already sees "another man, a phantom" made of her memories and readings, and no real man competes with him.
Around this axis run the other themes. Religion appears as one more source of sensation: after Rodolphe abandons her, Emma addresses God with "the same words of tenderness" she had murmured to her lover, and rises with "a vague feeling of an immense deception." The language is second-hand, that of novels, of official speeches, of Homais's newspaper, and the narrator goes so far as to say that "the human word is like a cracked cauldron." And money, through Lheureux's bills of exchange, accompanies every step of the adultery.
The ending closes the theme with irony. The novel does not end by punishing vice and rewarding virtue: it ends by rewarding Homais, the man who risked nothing for anyone, while Emma's love, those of the lovers, and Charles's are discarded or destroyed.