As emperor, Constantine enacted administrative, financial, social, and military reforms to strengthen the empire. He restructured the government, separating civil and military authorities. To combat inflation he introduced the solidus, a new gold coin that became the standard for Byzantine and European currencies for more than a thousand years. The Roman army was reorganised to consist of mobile field units and garrison soldiers capable of countering internal threats and barbarian invasions. Constantine pursued successful campaigns against the tribes on the Roman frontiers—the Franks, the Alamanni, the Goths, and the Sarmatians—even resettling territories abandoned by his predecessors during the Crisis of the Third Century.
Constantine was the first Roman emperor to convert to Christianity. Although he lived most of his life as a pagan, he joined the Christian faith on his deathbed, being baptised by Eusebius of Nicomedia. He played an influential role in the proclamation of the Edict of Milan in 313, which declared religious tolerance for Christianity in the Roman empire. He called the First Council of Nicaea in 325 that produced the statement of Christian belief known as the Nicene Creed. The Church of the Holy Sepulchre was built on his orders at the purported site of Jesus' tomb in Jerusalem and became the holiest place in Christendom. The Papal claim to temporal power in the High Middle Ages was based on the supposed Donation of Constantine. He is venerated as a saint by the Eastern Orthodox and Catholic Church. He has historically been referred to as the "First Christian Emperor," and he did heavily promote the Christian Church. Some modern scholars, however, debate his beliefs and even his comprehension of the Christian faith itself.
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 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 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 Constantine's reputation reflect the nature of the ancient sources for his reign. These are abundant and detailed, but have been strongly influenced by the official propaganda of the period, and are often one-sided. There are no surviving contemporaneous histories or biographies dealing with Constantine's life and rule. The nearest replacement is Eusebius's Vita Constantini, a work that is a mixture of eulogy and hagiography. Written between 335 AD and circa 339 AD, the Vita 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, the Origo 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 (408–50 AD), a century after Constantine's reign, these ecclesiastic 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 Arian Philostorgius also survive, though their biases are no less firm.