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.
History of Rome
Rome's population declined after its peak in the 2nd century. At the end of that century, during the reign of Marcus Aurelius, the Antonine Plague killed 2,000 people a day. Marcus Aurelius died in 180, his reign being the last of the "Five Good Emperors" and Pax Romana. His son Commodus, who had been co-emperor since AD 177, assumed full imperial power, which is most generally associated with the gradual decline of the Western Roman Empire. Rome's population was only a fraction of its peak when the Aurelian Wall was completed in the year 273 (in that year its population was only around 500,000).