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.
Holy Roman Emperor
The Emperor was crowned in a special ceremony, traditionally performed by the Pope in Rome. Without that coronation, no king, despite exercising all powers, could call himself Emperor. In 1508, Pope Julius II allowed Maximilian I to use the title of Emperor without coronation in Rome, though the title was qualified as "Electus Romanorum Imperator" ("elected Emperor of the Romans"). Maximilian's successors adopted the same titulature, usually when they became the sole ruler of the Holy Roman Empire. Maximilian's first successor Charles V was the last to be crowned Emperor.