The Roman Empire underwent considerable social, cultural and organizational changes starting with the reign of Diocletian (emperor 284 - 305 CE), who began the custom of splitting the Empire into Eastern and Western halves ruled by multiple emperors. Beginning with Constantine the Great, Christianity was made legal in the Empire, and a new capital was founded at Constantinople. Migrations of Germanic tribes disrupted Roman rule from the late 4th century onwards, culminating in the eventual collapse of the Empire in the West in 476, replaced by the so-called barbarian kingdoms. The resultant cultural fusion of Greco-Roman, Germanic and Christian traditions formed the foundations of the subsequent culture of Europe.
The general decline of population, technological knowledge and standards of living in Europe during this period became the archetypal example of societal collapse for writers from the Renaissance until recent times. As a result of this decline, and the relative scarcity of historical records from Europe in particular, the period from roughly the early fifth century until the Carolingian Renaissance (or later still) was once deprecatingly referred to as the "Dark Ages". This term has been abandoned by historians: in historical scholarship of the last half-century, the late West Roman Empire, the early Byzantine empire and the Early Middle Ages are increasingly being unified in the periodization of Late Antiquity.
The term Spätantike, literally "late antiquity", has been used by German-speaking historians since its popularization by Alois Riegl in the early 20th century. It was given currency in English partly by the writings of Peter Brown, whose survey The World of Late Antiquity (1971) revised the post-Gibbon view of a stale and ossified Classical culture, in favour of a vibrant time of renewals and beginnings, and whose The Making of Late Antiquity offered a new paradigm of understanding the changes in Western culture of the time in order to confront Sir Richard Southern's The Making of the Middle Ages.
The continuities between the later Roman Empire, as it was reorganized by Diocletian (r. 284–305), and the Early Middle Ages are stressed by writers who wish to emphasize that the seeds of medieval culture were already developing in the Christianized empire, and that they continued to do so in the Eastern Roman Empire or Byzantine Empire at least until the coming of Islam. Concurrently, some migrating Germanic tribes such as the Ostrogoths and Visigoths saw themselves as perpetuating the "Roman" tradition. While the usage "Late Antiquity" suggests that the social and cultural priorities of Classical Antiquity endured throughout Europe into the Middle Ages, the usage of "Early Middle Ages" or "Early Byzantine" emphasizes a break with the classical past, and the term "Migration Period" tends to de-emphasize the disruptions in the former Western Roman Empire caused by the creation of Germanic kingdoms within her borders beginning with the foedus with the Goths in Aquitania in 418.
A milestone in the rise of Christianity was the conversion of Emperor Constantine the Great (r. 306–337) in 312, as claimed by his Christian panegyrist Eusebius of Caesarea, although the sincerity of his conversion is debated. Constantine confirmed the legalization of the religion through the so-called Edict of Milan in 313, jointly issued with his rival in the East, Licinius (r. 308–324). By the late 4th century, Emperor Theodosius the Great had made Christianity the state religion, thereby transforming the Classical Roman world, which Peter Brown characterized as "rustling with the presence of many divine spirits."
Constantine I was a key figure in many important events in Christian history, as he convened and attended the first ecumenical council of bishops at Nicaea in 325, subsidized the building of churches and sanctuaries such as the Church of the Holy Sepulchre in Jerusalem, and involved himself in questions such as the timing of Christ's resurrection and its relation to the Passover.