Constantine was the first Roman emperor to convert to Christianity, and 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, at which the Nicene Creed was adopted by Christians. In military matters, 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.
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, and the later Eastern Empire was known as the Byzantine Empire. 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.
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 Eastern Orthodox and Byzantine Catholics. 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.