History of the East–West Schism
Disunion in the Roman Empire further contributed to disunion in the Church. The Emperor Diocletian famously divided the administration of the eastern and western portions of the Empire in the early 4th century, though subsequent leaders (including Constantine) aspired to and sometimes gained control of both regions. Theodosius the Great, who established Nicene Christianity as the official religion of the Roman Empire (see Edict of Thessalonica), was the last Emperor to rule over a united Roman Empire. Following his death in 395, the division into western and eastern halves, each for a few decades still under its own Emperor, was never reunited.
Byzantine Empire
Theodosius I (379–395) was the last Emperor to rule both the Eastern and Western halves of the Empire. In 391 and 392 he issued a series of edicts essentially banning pagan religion. Pagan festivals and sacrifices were banned, as was access to all pagan temples and places of worship. The last Olympic Games are believed to have been held in 393. In 395, Theodosius I bequeathed the imperial office jointly to his sons: Arcadius in the East and Honorius in the West, once again dividing Imperial administration. In the 5th century the Eastern part of the empire was largely spared the difficulties faced by the West – due in part to a more established urban culture and greater financial resources, which allowed it to placate invaders with tribute and pay foreign mercenaries. This success allowed Theodosius II to focus on the codification of Roman law and further fortification of the walls of Constantinople, which left the city impervious to most attacks until 1204. Large portions of the Theodosian Walls are preserved to the present day.