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.
Pax Romana
The Pax Romana (Latin for "Roman Peace") was a long period of relative peace and stability experienced by the Roman Empire. It is traditionally dated as commencing from the accession of Caesar Augustus, founder of the Roman principate, in 27 BCE and concluding in 180 CE with the death of Marcus Aurelius, the last of the "good emperors". Since it was inaugurated by Augustus with the end of the Final War of the Roman Republic, it is sometimes called the Pax Augusta. During this period of approximately 206 years, the Roman empire achieved its greatest territorial extent and its population reached a maximum of up to 70 million people – a third of the world’s population. According to Cassius Dio, the dictatorial reign of Commodus, later followed by the Year of the Five Emperors and the crisis of the third century, marked the descent "from a kingdom of gold to one of iron and rust".