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.
Marcus Aurelius
Marcus Aurelius ( or ; ; 26 April 121 – 17 March 180), called the Philosopher, was Roman emperor from . He ruled the Roman Empire with his adoptive brother, Lucius Verus, until Lucius' death in 169. He was the last of the rulers traditionally known as the Five Good Emperors. He is also seen as the last emperor of the Pax Romana, an age of relative peace and stability for the Empire.