Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
"Dear Miss Morland, consider the dreadful nature of the suspicions you have entertained. What have you been judging from? Remember the country and the age in which we live. Remember that we are English, that we are Christians. Consult your own understanding, your own sense of the probable, your own observation of what is passing around you. Does our education prepare us for such atrocities? Do our laws connive at them? Could they be perpetrated without being known, in a country like this, where social and literary intercourse is on such a footing, where every man is surrounded by a neighbourhood of voluntary spies, and where roads and newspapers lay everything open? Dearest Miss Morland, what ideas have you been admitting?"
They had reached the end of the gallery, and with tears of shame she ran off to her own room.
— Jane Austen, Northanger Abbey
Like Atonement, Jane Austen's Northanger Abbey was a novel about novels. In particular, it was a satire of the Gothic novels popular at the time (1803). In these stories, villainous uncles imprisoned their nieces, husbands murdered wives, and grisly corpses leaned out of closets to shout "booga-booga!"
Catherine, who is mentioned in the epigraph, is Austen's heroine, and she reads bunches and bunches of novels by people like Anne Radcliffe—sort of the Stephen King of her day. Catherine has read so many books about murder and rape and pillage that she ends up deciding that her boyfriend's father has imprisoned or murdered his wife!
He hasn't. Her boyfriend, Mr. Tilney (who speaks the epigraph), discovers her suspicions and tells her she is a dope. Much embarrassment ensues.
The parallel with the plot of Atonement is fairly straightforward. Like Catherine, Briony has false suspicions. And like Catherine, Briony sheds "tears of shame"—in this case, the novel itself—when she discovers her error.
There are some contrasts between Atonement and Northanger Abbey, too. Northanger Abbey is a comedy—Catherine's suspicions are not believed, and no ill comes of them. Briony, on the other hand, destroys multiple lives because of her foolishness.
In addition, Northanger Abbey, in that quote, is very sure of the propriety of the English. "Remember that we are English, that we are Christians… Does our education prepare us for such atrocities? Do our laws connive at them?" Northanger Abbey'sanswer to these questions is, no, the English do not connive at atrocities, and indeed that such atrocities don't really exist.
Atonement is not so sure. On the contrary, the book is filled with atrocities, including rape, false imprisonment, and war. Crimes can absolutely "be perpetrated without being known, in a country like this, where social and literary intercourse is on such a footing." From the perspective of Atonement, then, it's not just Catherine who is naïve, but Mr. Tilney too.
So the epigraph gets us set to hear about making things up and feeling embarrassed and Englishness. It also tells us that we're coming into a British novel about British novels, and that much Britishness and novelness is going to ensue.