The Imperial cult of ancient Rome identified emperors and some members of their families with the divinely sanctioned authority (auctoritas) of the Roman State. Its framework was based on Roman and Greek precedents, and was formulated during the early Principate of Augustus. It was rapidly established throughout the Empire and its provinces, with marked local variations in its reception and expression.
Augustus's reforms transformed Rome's Republican system of government to a de facto monarchy, couched in traditional Roman practices and Republican values. The princeps (later known as Emperor) was expected to balance the interests of the Roman military, Senate and people, and to maintain peace, security and prosperity throughout an ethnically diverse empire. The official offer of cultus to a living emperor acknowledged his office and rule as divinely approved and constitutional: his Principate should therefore demonstrate pious respect for traditional Republican deities and mores.
A deceased emperor held worthy of the honor could be voted a state divinity (divus, plural divi) by the Senate and elevated as such in an act of apotheosis. The granting of apotheosis served religious, political and moral judgment on Imperial rulers and allowed living Emperors to associate themselves with a well-regarded lineage of Imperial divi from which unpopular or unworthy predecessors were excluded. This proved a useful instrument to Vespasian in his establishment of the Flavian Imperial Dynasty following the death of Nero and civil war, and to Septimius in his consolidation of the Severan dynasty after the assassination of Commodus.
The Imperial cult was inseparable from that of Rome's official deities, whose cult was essential to Rome's survival and whose neglect was therefore treasonous. Traditional cult was a focus of Imperial revivalist legislation under Decius and Diocletian. Christian apologists and martyrologists saw the cult of the Emperor as a particularly offensive instrument of pagan impiety and persecution. It therefore became a focus of theological and political debate during the ascendancy of Christianity under Constantine I. The emperor Julian failed to reverse the declining support for Rome's official religious practices: Theodosius I adopted Christianity as Rome's state religion. Rome's traditional gods and Imperial cult were officially abandoned. However, many of the rites, practices and status distinctions that characterized the cult to emperors were perpetuated in the theology and politics of the Christianized Empire.
For five centuries, the Roman Republic did not give worship to any historic figure, or any living man, although surrounded by divine and semi-divine monarchies. Rome's legendary kings had been its masters; with their removal, Republican Romans could identify Romulus, the founder of the city, with the god Quirinus and still retain Republican liberty. Similarly, Rome's ancestor-hero Aeneas was worshipped as Jupiter Indiges. The Romans worshipped several gods and demi-gods who had been human, and knew the theory that all the gods had originated as human beings, yet Republican traditions (mos maiorum) were staunchly conservative and anti-monarchic. The aristocrats who held almost all Roman magistracies, and thereby occupied almost all of the Senate, acknowledged no human as their inherent superior. No citizen, living or dead, was officially regarded as divine, but the honors awarded by the state — crowns, garlands, statues, thrones, processions — were also suitable to the gods, and tinged with divinity; indeed, when the emperors were later given state worship, it was done by a decree of the Senate, phrased like any other honor.
Among the highest of honors was the triumph. When a general was acclaimed imperator by his troops, the Senate would then choose whether to award him a triumph, a parade to the Capitol in which the triumphator displayed his captives and spoils of war in the company of his troops; by law, all were unarmed. The triumphator rode in a chariot, bearing divine emblems, in a manner supposed to be inherited from the ancient kings of Rome, and ended by dedicating his victory to Jupiter Capitolinus. Some scholars have viewed the triumphator as impersonating or even becoming a king or a god (or both) for the day but the circumstances of triumphal award and subsequent rites also functioned to limit his status. Whatever his personal ambitions, his victory and his triumph alike served the Roman Senate, people, and gods and were recognised only through their consent.
In private life, however, tradition required that some human beings be treated as more or less divine; cult was due from familial inferiors to their superiors. Every head of household embodied the genius – the generative principle and guardian spirit – of his ancestors, which others might worship and by which his family and slaves took oaths; his wife had a juno. A client could call his patron "Jupiter on earth". The dead, collectively and individually, were gods of the underworld or afterlife (dii manes). A letter has survived from Cornelia, the mother of the Gracchi, expecting that when she was dead, her sons would venerate her as deus parens, a parental (or a nurturing) divinity; such piety was expected from any dutiful son.