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.
Roman emperor
This line of Roman emperors was actually generally Germanic rather than Roman, but maintained their Roman-ness as a matter of principle. These emperors used a variety of titles (most frequently ""Imperator Augustus"") before finally settling on "Imperator Romanus Electus" ("Elected Roman Emperor"). Historians customarily assign them the title "Holy Roman Emperor", which has a basis in actual historical usage, and treat their "Holy Roman Empire" as a separate institution. To Latin Catholics of the time, the Pope was the temporal authority as well as spiritual authority, and as Bishop of Rome he was recognized as having the power to anoint or crown a new Roman emperor. The last man to be crowned by the pope (although in Bologna, not Rome) was Charles V. All his successors bore only a title of "Elected Roman Emperor".