One of the foremost scholars of Constantine's reign seeks to improve on Edward Gibbon's eighteenth-century ambition to achieve a "just portrait" of Constantine. In contrast to most writing on Constantine throughout the centuries, Barnes' interpretation describes an emperor who was neither tyrant nor saint, who established Christianity as the official religion of the empire by suppressing Jews and pagans, who preserved and modified rather than revolutionized the imperial office and its traditions (with the notable exception in connection with pagan sacrifice, which he totally abolished). In effect, Constantine emerges as a genuine, but flawed Christian convert who remained a typical Roman emperor but who was driven by the conviction that he was divinely appointed to convert the Empire to Christianity.
Barnes also plunges the full range of works by Eusebius of Caesarea, producing a nuanced portrait of our main source on Constantine as well as the emperor himself. Multiple myths about both men are dispelled, not least those which each man preferred others to believe about himself.
While it is a work with which any student of late antiquity in general or Constantine in particular needs to be well-acquainted with, Barnes' taste for nuance does not avoid the exaggeration that Constantine established Christianity as the official religion (that was his successor, Theodosius I, in 381, who was more radical than Constantine ever was). He also fails to deal with or integrate his interpretation in light of the lack of a distinction between religious and secular matters, a major of fault of perhaps all interpreters of Constantine since the Enlightenment - one which seems all but inescapable for contemporary scholars.
Unlike many contemporary writers, Barnes integrates theology into history with a level of competence surprising for a non-Christian, non-theologian. History and theology are inextricable in much of Barnes' account, and the author recognizes how history and theology drive one another during the emperor's reign.
See Barnes' 2013 follow-up entitled Constantine: Dynasty, Religion and Power in the Later Roman Empire for both a self-critique and affirmation of this earlier major work from 1981.