The age of Constantine marked a distinct epoch in the history of the Roman Empire. He built a new imperial residence at Byzantium and renamed the city Constantinople (now Istanbul) after himself (the laudatory epithet of "New Rome" came later, and was never an official title). It became the capital of the Empire for more than a thousand years, with the later eastern Roman Empire, now being referred to as the Byzantine Empire by historians. His more immediate political legacy was that he replaced Diocletian's tetrarchy with the principle of dynastic succession by leaving the empire to his sons. His reputation flourished during the lifetime of his children and for centuries after his reign. The medieval church upheld him as a paragon of virtue, while secular rulers invoked him as a prototype, a point of reference, and the symbol of imperial legitimacy and identity. Beginning with the Renaissance, there were more critical appraisals of his reign, due to the rediscovery of anti-Constantinian sources. Trends in modern and recent scholarship have attempted to balance the extremes of previous scholarship.
Constantine was a ruler of major importance, and he has always been a controversial figure. The fluctuations in his reputation reflect the nature of the ancient sources for his reign. These are abundant and detailed, but they have been strongly influenced by the official propaganda of the period and are often one-sided; no contemporaneous histories or biographies dealing with his life and rule have survived. The nearest replacement is Eusebius's Vita Constantini—a mixture of eulogy and hagiography written between 335 AD and circa 339 AD—that extols Constantine's moral and religious virtues. The Vita creates a contentiously positive image of Constantine, and modern historians have frequently challenged its reliability. The fullest secular life of Constantine is the anonymous Origo Constantini, a work of uncertain date, which focuses on military and political events to the neglect of cultural and religious matters.
Lactantius' De Mortibus Persecutorum, a political Christian pamphlet on the reigns of Diocletian and the Tetrarchy, provides valuable but tendentious detail on Constantine's predecessors and early life. The ecclesiastical histories of Socrates, Sozomen, and Theodoret describe the ecclesiastic disputes of Constantine's later reign. Written during the reign of Theodosius II (408–450 AD), a century after Constantine's reign, these ecclesiastical historians obscure the events and theologies of the Constantinian period through misdirection, misrepresentation, and deliberate obscurity. The contemporary writings of the orthodox Christian Athanasius and the ecclesiastical history of the ArianPhilostorgius also survive, though their biases are no less firm.