From its foundation by Augustus Caesar in 27 b.c.e., the Roman Empire saw a tendency to treat the emperor as a divine being. This phenomenon was neither completely government imposed nor entirely a spontaneous upwelling of devotion from the people of the empire. The divinization of the emperor was only partially reversed with the conversion of the empire to Christianity.
Augustus himself built on elements in the earlier rule of his uncle Julius Caesar, who the Roman Senate declared a god posthumously with Augustus’s influence. Like Julius, Augustus took the ancient title of pontifex maximus, chief priest of the Roman state, a title that subsequent emperors would also take as part of their office.
This is consistent with Augustus’s general practice of using titles either sanctioned by Roman tradition or relatively modest, such as princeps, or first citizen. Augustus’s reign saw the foundation of temples of "Rome and Augustus" throughout the empire. More temples of Augustus survive than of any other emperor, including the only imperial temples found outside the boundaries of the empire.
The cult paid to the living Augustus was more like that of heroes and benefactors than that of the actual gods and was particularly strong in the Greek East, where it built on Greek and Near Eastern traditions of ruler cults.
However, after his death in 14 c.e. the Roman Senate enrolled divus Augustus, divine Augustus, among the gods of the Roman state (following the precedent of Julius). There is also evidence for cults of other members of his family.
Augustus’s successor, Tiberius, encouraged his cult, sponsoring an official priesthood, although he himself declined divine honors when proffered. He and all subsequent emperors also took the names Caesar Augustus.
The third emperor of the Julio-Claudian line, Gaius, or Caligula, contrasted with Tiberius in his lust for divine honors. A lust to be paid divine honors while still alive became a stereotypical quality of "bad" emperors in the writings of Roman historians.
Enrollment of a dead emperor among the gods of Rome was an official act of the Senate and as such was a political statement about the merits of the dead emperor. If a new emperor wanted to emphasize continuity, having the Senate declare his predecessor divine was an effective means.
If a new emperor wanted to emphasize a sharp break with the past, leaving the dead emperor undeified helped send that message. After the death of the emperor Claudius—the conqueror of Britain—a magnificent temple, the largest on the island, was begun at Colchester dedicated to the divine emperor. It was later destroyed by British rebels, led by Boudicca.
The cult played a key role in Roman persecution of Christians, who were urged to perform a sacrifice to the emperor to avoid punishment. The most thorough persecution of the Christians, perpetrated by the emperor Diocletian, was part of an attempt to strengthen the imperial cult as a way of unifying the empire. Jews were usually exempt from this requirement on the condition that they prayed to their god for the emperor.
Diocletian also reformed the protocol surrounding the emperor to make himself more remote from ordinary people, and abandoned traditional titles like princeps in favor of the more arrogant Dominus Noster, or "Our Lord". Historians dispute whether the imperial cult was purely political, or whether the feelings it evoked were religious like those of the gods and heroes.
Although the imperial cult varied tremendously between regions, there is some evidence that it was integrated into local religious life. Local associations could choose emperors or members of the imperial family as their divine patrons.
After the conversion of Constantine the Great to Christianity, the emperor abandoned his pagan religious role and took on a new one as protector and arbiter of the Christian Church.
The abandonment of the imperial cult was a slow process; the Christian emperors continued to use the title pontifex maximus until Gratian renounced it as part of a campaign against the pagan high aristocracy of the city of Rome. There are also some signs of the persistence of imperial priesthoods in the fourth century c.e. under Christian emperors.
The emperors themselves were not ordinary Christians but retained an exalted and sacred status. The peculiar religious position of the Byzantine emperors, whose titles included isapostolon, or "equal of the apostles", was partially an inheritance from the pagan Roman imperial cult.