Consider the case of a privileged young man, promiscuous, pompous, and prevaricating. The sinner marries, produces a son, and sends his wife away when she proves an encumbrance to his newfound career as a Christian. Approaching early middle age, ordained in rather irregular circumstances, this bishop - an ambitious writer, one with pretensions to literary greatness - confounds the skeptics by seeming to bare all. In one masterstroke the spiritual autobiography he authors redeems his reputation. Sinning comes to seem the inevitable avenue to salvation. His unabashed confession of his corruption wins hearts.

James O'Donnell positively revels in making Augustine our contemporary, a guy whose autobiography is as disarming as Huck Finn's. Indeed, the biographer begins his chapter "Augustine Confesses" with this gambit: "Huck Finn's words are worth heeding." Paraphrasing Twain, the biographer then begins his analysis of Augustine's veracity: "You probably don't know about Augustine without you have read, or at least heard about, the book called his 'Confessions'" - that is, "mostly a true book, with some stretchers."

Mr. O'Donnell's desire to make Augustine one of us is no quirk. In 1933, Rebecca West published a magnificent short biography, which hailed her subject as "one of the greatest of all writers [who] works in the same introspective field as the moderns. In his short violent sentences, which constantly break out in the rudest tricks of the rhetoricians, rhymes, puns, and assonances, he tries to do exactly what Proust tried to do in his long, reflective sentences ... He tries to take a cast of his mental state at a given moment."

Before his conversion to Christianity, Augustine resembled a dandy of somewhat dissolute habits, a professor as public intellectual who commanded high speaking fees. Or, as Mr. O'Donnell writes, "The young Augustine in Milan, awash in flop sweat in his stretch limousine en route to praise the emperor or his latest favorite, had made his way in the world as a prizewinning stage performer of this sort." What is striking about Augustine, of course, is that he gave up the life of celebrity to return to his African origins - to Hippo, in his time only about one-tenth the size of Carthage.

Even more amazingly, Augustine took command of a church dwarfed in size and prestige by that of his arch rivals, the Donatists, who preached a strain of puritanism that would have excluded the young Augustine (for example) from their ranks. Augustine countered, in effect, that you could not have a Christian church without sinners. By 411 he had triumphed over the Donatist majority.

Augustine attacked superficial notions of free will and the Donatist idea that living a pure life was the key to salvation. He favored exposing the complex and often self-deceptive psychology of believers. Such an attitude makes him the extraordinarily modern figure that attracted West, and Mr. O'Donnell portrays with a zest that nearly matches hers.

Here is Mr. O'Donnell's introduction to his analysis of the "Confessions": "The man with the voice unlike any other was never more onstage than when he set out to reveal himself to us." In other words, the very act of confession is a performance that conceals as much as it discloses. That Augustine told stretchers is not so much a criticism of the man as an acknowledgment that Mr. O'Donnell, like Huck Finn, "never seen anybody but lied one time or another."

I confess that, after reading the incomparable Rebecca West, other accounts of Augustine have always bored me - even the standard "Augustine of Hippo" by Peter Brown and Garry Wills's more recent Penguin life. But in her short biography, West could not do justice to all of Augustine's work, especially his "City of God." For including an in-depth analysis of Augustine's masterwork, and for fresh translations of Augustine's writings, we are deeply indebted to Mr. O'Donnell, who is also editor of the definitive three-volume edition of the "Confessions."

If Mr. O'Donnell's biography cannot be called definitive, he himself provides the reason. In 2000 Pierre-Marie Hombert published a powerful study, "New Investigations in Augustinian Chronology," which called into question the sequence of events that transformed Augustine into a "significant and recognized figure in the African church and beyond." The creation of this "Augustine" took much longer than has been traditionally supposed. In effect, Hombert challenged all previous accounts of the central phase of Augustine's career.

It will take generations to digest Hombert's challenge to Augustinian biography, Mr. O'Donnell suggests - adding that the "study of Augustinian chronology, and thus all of Augustine's life, is built on shaky ground." Thus we must approach Mr. O'Donnell's own work with, in his words, "suspended confidence." Perhaps so. But in the light of his own shrewd reading of his sources, I would contend that what future biographers make of Augustine's career will, in part, be built on Mr. O'Donnell's own splendidly probing narrative.