Atonement is self-referential… which is a fancy way of saying that it's a book that likes to talk about itself. You can tell this because the biggest, most honking, I-am-a-symbol symbol in the book is the play, The Trials of Arabella. And what does the symbol symbolize? The book you're reading!
In the first paragraph of the book (1.1.1) we see Briony writing her play, The Trials of Arabella, in "a two-day tempest of composition, causing her to miss breakfast and lunch." In the last pages of the book, Briony finally sees her play performed, some sixty-five years later. Completing the play, then, lasts as long as the book. To get to the end of the one, you've got to get to the end of the other.
And there's more. The plot of The Trials of Arabella has some suspicious similarities to the book you're reading. Arabella, the princess, falls in love with an impoverished doctor—just like Cecilia falls in love with Robbie, who is (ta-daaa!) an impoverished doctor (okay, he's an aspiring doctor, but close enough). The play also features "an impetuous dash toward a seaside town"—and the second part of the novel is devoted to Robbie's dash (not so impetuous) to Dunkirk during the war.
The biggest parallel between play and book, though, is helpfully spelled out for you at the end of the book (thanks Ian McEwan). The play has a happy ending:
Here's the beginning of our love at the end of our travail. / So farewell, kind friends, as into the sunset we sail! (4.41)
Briony, at seventy-seven, after watching the performance, has a little epiphany. "It occurs to me that I have not traveled so very far after all, since I wrote my little play"(4.45), she says. She still wants a happy ending, no matter how contrived, or forced. So she gives Cecilia and her doctor one, just as she gave Arabella and her doctor one. The Trials of Arabella is just like Atonement—except for the ways that they're not. If a theme of Atonement is that fiction and reality don't quite line up, then it makes sense that the symbol and the thing symbolized won't quite match either. Which is why the novel's not called The Trials of Arabella. (Okay, also probably because Atonement sounds snappier.)