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.
Last Roman Emperor
The legend is based on the "Apocalypse of Pseudo-Methodius", which was, after the Book of Daniel and the Book of Revelation, "the most widespread apocalypse story in Europe". It proposes a different Last Emperor from the previous figure, who fought against religious enemies, and introduced the conflict with Islam: he will "go forth against them [the enemies of the faith] from the Ethiopian sea and will send the sword and desolation into Ethribus their homeland, capturing their women and children living in the Land of Promise". After conquering his enemies he would travel to Jerusalem and relinquish his power on the Mount of Olives. The Last Emperor was further developed in the writings of Adso of Montier-en-Der, whose "Libellus de Antichristo" (ca. 954) was a popular biography of the Antichrist, whose coming was preceded by the rise of a Frankish ruler (the continuation of the Roman Empire); this Last Emperor would voluntarily give up his power and die, after which the Antichrist comes to power. Another important impetus came from the oracles of the Tiburtine Sibyl, first recorded in Latin around the year 1000; its legend proved particularly adaptable to rulers all over Europe, containing as it did a list of emperors and kings leading up to the Last Emperor which could be revised or added to as political and dynastic circumstances required. It still had great currency in the fifteenth century.