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.
Prince-elector
From the 13th century onwards, the Prince-Electors had the privilege of electing the King of the Romans, who would be crowned by the Pope as Holy Roman Emperor. Charles V was the last to be a crowned Emperor (elected 1519, crowned 1530); his successors were elected Emperors directly by the electoral college, each being titled "Elected Emperor of the Romans" (; ). In practice, every emperor from 1440 onwards (except for Charles VII and Francis I) came from the Austrian House of Habsburg, and the Electors merely ratified the Habsburg succession.