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.
Legacy of the Roman Empire
From a legal point of view the Roman Empire, founded by Augustus in 27 BC and divided into two "parts" after the death of Theodosius I in 395, had survived only in the eastern part which, with the deposition of the last western emperor Romulus Augustulus, in 476, had also obtained the imperial regalia of the western part reuniting from a formal point of view the Roman Empire.