Culture of ancient Rome
The Roman Empire, at its height (c. 117 CE), was the most extensive political and social structure in western civilization. By 285 CE the empire had grown too vast to be ruled from the central government at Rome and so was divided by Emperor Diocletian into a Western and an Eastern Empire. The Roman Empire began when Augustus Caesar became the first emperor of Rome (31 BCE) and ended, in the west, when the last Roman emperor, Romulus Augustulus, was deposed by the Germanic King Odoacer (476 CE). In the east, it continued as the Byzantine Empire until the death of Constantine XI and the fall of Constantinople to the Ottoman Turks in 1453 CE. The influence of the Roman Empire on western civilization was profound in its lasting contributions to virtually every aspect of western culture.
Holy Roman Emperor
The Holy Roman Emperor was widely perceived to rule by divine right, though he often contradicted or rivaled the Pope, most notably during the Investiture controversy. 
In theory, the Holy Roman Emperor was "primus inter pares" (first among equals) among other Catholic monarchs. In practice, a Holy Roman Emperor was only as strong as his army and alliances, including marriage alliances, made him. There was never a Holy Roman Empress regnant, though women such as Theophanu and Maria Theresa of Austria served as de facto Empresses regnant.
Throughout its history, the position was viewed as a defender of the Roman Catholic faith. Until the Reformation, the Emperor elect ("imperator electus") was required to be crowned by the Pope before assuming the imperial title. Charles V, Holy Roman Emperor was the last to be crowned by the Pope in 1530. Even after the Reformation, the elected Emperor always was a Roman Catholic. There were short periods in history when the electoral college was dominated by Protestants, and the electors usually voted in their own political interest.