Roman emperor
The Western Roman Empire collapsed in the late 5th century. Romulus Augustulus is often considered to be the last emperor of the West after his forced abdication in 476, although Julius Nepos maintained a claim recognized by the Eastern Empire to the title until his death in 480. Following Nepos' death, the Eastern Emperor Zeno abolished the division of the position and proclaimed himself as the sole Emperor of a reunited Roman Empire. The Eastern imperial lineage continued to rule from Constantinople ("New Rome"); they continued to style themselves as Emperor of the Romans (later βασιλεύς Ῥωμαίων in Greek), but are often referred to in modern scholarship as Byzantine emperors. Constantine XI Palaiologos was the last Roman emperor in Constantinople, dying in the Fall of Constantinople to the Ottomans in 1453.
Christianity in the 4th century
Disunion in the Roman Empire further contributed to disunion in the Church. Emperor Diocletian 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 I, who established Christianity as the official religion of the Roman Empire, died in 395 and was the last emperor to rule over a united Roman Empire; following his death, the division into western and eastern halves, each under its own emperor, became permanent. By the end of the 5th century, the Western Roman Empire had been overrun by the Germanic tribes, while the Eastern Roman Empire (known also as the Byzantine Empire) continued to thrive. Thus, the political unity of the Roman Empire was the first to fall.