Before novels were a thing, works didn't have titles so much as descriptions. Think of books like Beowulf (the story of Beowulf), the Odyssey (the story of Odysseus), the Aeneid (the story of Aeneas)—you get the drift. These aren't 20th-century titles like For Whom the Bell Tolls or As I Lay Dying. If Augustine had written his book now, it might have been called something like Sin Pray Love. We should also add that, in a bookstore, you might find this book under The Confessions of Saint Augustine or Augustine's Confessions, but really the book would have just been called Confessions… with Augustine listed as the author.
So. Is there anything to analyze in this one-word title? Well, since the book is one long series of confessions, we know that the speaker of the book has done something very, very bad. After all, you don't "confess" your good deeds. So, in calling the work his "confessions," the first thing that we know about Augustine is that he is a sinner—though what exactly he has done, we have yet to be told, and now we really want to know. We also know that he wants to confess, which is telling, too. You usually aren't too eager to blab about something horrible you did unless there's a good reason to blab. Like the fact that your soul is in crisis.
In fact, the very need to confess usually implies that whatever bad thing you have done is eating away at you. So you speak (or write) your sins, because this act is somehow redeeming. Hm, we wonder where Augustine might have gotten an idea like that…?
In the centuries since the Confessions, there have been a slew of other "confessions" books. Often these books have characters who have done things far, far worse than your average, everyday sinner like Augustine. In fact, more often than not, these books don't have a religious or moralistic outcome. Take Confessions of an English Opium-Eater (1821) by Thomas de Quincy, or The Private Memoirs and Confessions of a Justified Sinner (1824) by James Hogg, or even the film Confessions of a Dangerous Mind (2002). When people talk about the beginning of confessional novels, maybe they should start considering our man Augustine as breaking some original ground.