From the time of Constantine I (r. 306–337), the Roman emperors had, with very few exceptions, taken on a role as promoters and defenders of Christianity.
The reign of Constantine established a precedent for the position of the Christian emperor in the Church. Emperors considered themselves responsible to the gods for the spiritual health of their subjects, and after Constantine they had a duty to help the Church define orthodoxy and maintain orthodoxy. The emperor's role was to enforce doctrine, root out heresy, and uphold ecclesiastical unity.
Both the title and connection between Emperor and Church continued in the Eastern Roman Empire throughout the medieval period (in exile during 1204–1261). The ecumenical councils of the 5th to 8th centuries were convoked by the Eastern Roman Emperors.
In Western Europe, the title of Emperor became defunct after the death of Julius Nepos in 480, although the rulers of the barbarian kingdoms continued to recognize the authority of the Eastern Emperor at least nominally well into the 6th century. In 797, the Eastern Emperor Constantine VI was deposed and replaced as monarch by his mother, Irene. The Papacy, which up until this point had continued to recognize the rulers in Constantinople as Roman Emperors, viewed the imperial throne as vacant since in their mind, a woman could not rule the empire.
For this reason, Charlemagne, the King of the Franks and King of Italy, was crowned Emperor of the Romans (Imperator Romanorum) by Pope Leo III, as the successor of Constantine VI as Roman Emperor under the concept of translatio imperii. The Eastern Empire eventually relented to recognizing Charlemagne and his successors as emperors, but as "Frankish" and "German emperors", at no point referring to them as Roman, a label they reserved for themselves.
The title of Emperor in the West implied recognition by the pope. As the power of the papacy grew during the Middle Ages, popes and emperors came into conflict over church administration. The best-known and most bitter conflict was that known as the investiture controversy, fought during the 11th century between Henry IV and Pope Gregory VII.
After the coronation of Charlemagne, his successors maintained the title until the death of Berengar I of Italy in 924. The comparatively brief interregnum between 924 and the coronation
of Otto the Great in 962 is taken as marking the transition from the Frankish Empire to the Holy Roman Empire.
Under the Ottonians, much of the former Carolingian kingdom of Eastern Francia fell within the boundaries of the Holy Roman Empire.
Since 911, the various German princes had elected the King of the Germans from among their peers. The King of the Germans would then be
crowned as emperor following the precedent set by Charlemagne, during the period of 962–1530. Charles V was the last emperor to be crowned by the pope, and his successor, Ferdinand I, merely adopted the title of "Emperor elect" in 1558. The final Holy Roman Emperor-elect, Francis II, abdicated in 1806 during the Napoleonic Wars that saw the Empire's final dissolution.
The term sacrum (i.e., "holy") in connection with the German Roman Empire was first used in 1157 under Frederick I Barbarossa.
The standard designation of the Holy Roman Emperor was "August Emperor of the Romans" (Romanorum Imperator Augustus). When Charlemagne was crowned in 800, he was styled as "most serene Augustus, crowned by God, great and pacific emperor, governing the Roman Empire," thus constituting the elements of "Holy" and "Roman" in the imperial title.
The word Roman was a reflection of the principle of translatio imperii (or in this case restauratio imperii) that regarded the (Germanic) Holy Roman Emperors as the inheritors of the title of Emperor of the Western Roman Empire, despite the continued existence of the Eastern Roman Empire.
In German-language historiography, the term Römisch-deutscher Kaiser ("Roman-German emperor") is used to distinguish the title from that of Roman Emperor on one hand, and that of German Emperor (Deutscher Kaiser) on the other. The English term "Holy Roman Emperor" is a modern shorthand for "emperor of the Holy Roman Empire" not corresponding to the historical style or title, i.e., the adjective "holy" is not intended as modifying "emperor"; the English term "Holy Roman Emperor" gained currency in the interbellum period (1920s to 1930s); formerly the title had also been rendered "German-Roman emperor" in English.