Apollo and Daphne, marble sculpture by Gian Lorenzo Bernini, 1622–24; in the Borghese Gallery, Rome,

For the earliest times, there are the various finds and findings of archaeology. But they are not sufficient to enable scholars to reconstruct archaic Roman religion. They do, however, suggest that early in the 1st millennium BC, though not necessarily at the time of the traditional date for the founding of Rome (753 BC), Latin and Sabine shepherds and farmers with light plows came from the Alban Hills and the Sabine Hills, and that they proceeded to establish villages at Rome, the Latins on the Palatine Hill and the Sabines (though this is uncertain) on the Quirinal and Esquiline hills. About 620 the communities merged, and c. 575 the Forum Romanum between them became the town’s meeting place and market.

From such evidence it appears that the early Romans, like many other Italians, sometimes saw divine force, or divinity, operating in pure function and act, such as in human activities like opening doors or giving birth to children, and in nonhuman phenomena such as the movements of the sun and seasons of the soil. They directed this feeling of veneration both toward happenings that affected human beings regularly and, sometimes, toward single, unique manifestations, such as a mysterious voice that once spoke and saved them in a crisis (Aius Locutius). They multiplied functional deities of this kind to an extraordinary degree of “religious atomism,” in which countless powers or forces were identified with one phase of life or another. Their functions were sharply defined; and in approaching them it was important to use their right names and titles. If one knew the name, one could secure a hearing. Failing that, it was often best to cover every contingency by admitting that the divinity was “unknown” or adding the precautionary phrase “or whatever name you want to be called” or “if it be a god or goddess.”

The same sort of anxious awe was extended not only to functions and acts but also to certain objects that inspired a similar belief that they were in some way more than natural. This feeling was aroused, for example, by springs and woods, objects of gratitude in the torrid summer, or by stones that were often believed to be meteorites — i.e., had apparently reached the earth in an uncanny fashion. To these were added products of human action, such as burial places and boundary stones, and inexplicable things, such as Neolithic implements (probably the mysterious meteorites were often these) or bronze shields (artifacts that had strayed in from more advanced cultures).

To describe the powers in these objects and functions that inspired the horror, or sacred thrill, the Romans eventually employed the word numen, suggestive of a god’s nod, nutus; though so far there is no evidence that this usage was earlier than the 2nd century BC. The application of the word spirit to numen is anachronistic in regard to early epochs because it presupposes a society capable of greater abstraction. Nor must the term mana, used by Melanesians to describe their own concept of superhuman forces, be introduced too readily. The two societies are not necessarily analogous and, besides, the deduction from such comparisons that the Romans experienced an impersonal, pre-deistic, primordial stage of religion that neatly preceded the personal stage cannot be regarded as correct. On the contrary, from the very earliest times, the supernatural forces that they envisaged included a number of deities in analogous human forms; among them were certain “high gods.” Foremost among these was a divinity of the sky, Jupiter, akin to the sky gods of other early Indo-European-speaking peoples, the Sanskrit Dyaus and Greek Zeus. Not yet, probably, a Supreme Being, though superior in some sense to other divine powers, this god of the heavens was easily linked with the forces of function and object, with lightning and weather, or with the uncanny stone that came from on high and was called Jupiter Lapis.

These gods and sacred functions and objects seemed charged with power because they were mysterious and alarming. In order to secure their food supply, physical protection, and growth in numbers, the early Romans believed that such forces had to be propitiated and made allies. Sacrifice was necessary. The product sacrificed would revitalize the divinity, which was seen as a power of action and therefore likely to run down unless so revitalized. By this nourishment he or it would become able and ready to fulfill requests. And so the sacrifice was accompanied by the phrase macte esto! (“be you increased!”).

Prayer was a normal accompaniment of sacrifice, and as a conception of the divine powers gradually developed, it contained varying ingredients of flattery, cajolery, and attempted justification; but it also was compounded by magic — the attempt not to persuade nature, but to coerce it. Though the authorities (e.g., c. 451-450 BC, Law of the Twelve Tables) sought to limit its noxious aspects, magic continued to abound throughout the ancient world. Even official rites remained full of its survivals, notably the annual festival of the Lupercalia and the ritual dances of the Salii in honour of Mars. Romans in historical times regarded magic as an oriental intrusion, but Italian tribes, such as the Marsi and Paeligni, were famous for such practices. Among them curses figured prominently, and curse inscriptions from c. 500 BC onward have been found in large numbers. There were also numerous survivals of taboo, a negative branch of magic: people were admonished to have no dealings with strangers, corpses, newborn children, spots struck by lightning, etc., lest harm would befall them.

Roman investigations into early Christianity found it an irreligious, novel, disobedient, even atheistic sub-sect of Judaism: it appeared to deny all forms of religion and was therefore superstitio. By the end of the Imperial era, Nicene Christianity was the one permitted Roman religio; all other cults were heretical or pagan superstitiones.

After the Great Fire of Rome in 64 AD, Emperor Nero accused the Christians as convenient scapegoats, who were later persecuted and killed. From that point on, Roman official policy towards Christianity tended towards persecution.

During the various Imperial crises of the 3rd century, "contemporaries were predisposed to decode any crisis in religious terms", regardless of their allegiance to particular practices or belief systems. Christianity drew its traditional base of support from the powerless, who seemed to have no religious stake in the well-being of the Roman State, and therefore threatened its existence. The majority of Rome's elite continued to observe various forms of inclusive Hellenistic monism; Neoplatonism in particular accommodated the miraculous and the ascetic within a traditional Graeco-Roman cultic framework. Christians saw these practices as ungodly, and a primary cause of economic and political crisis.

In the wake of religious riots in Egypt, the emperor Decius decreed that all subjects of the Empire must actively seek to benefit the state through witnessed and certified sacrifice to "ancestral gods" or suffer a penalty: only Jews were exempt. Decius' edict appealed to whatever common mos maiores might reunite a politically and socially fractured Empire and its multitude of cults; no ancestral gods were specified by name. The fulfillment of sacrificial obligation by loyal subjects would define them and their gods as Roman. Apostasy was sought, rather than capital punishment. A year after its due deadline, the edict expired.

Valerian singled out Christianity as a particularly self-interested and subversive foreign cult, outlawed its assemblies and urged Christians to sacrifice to Rome's traditional gods. In another edict, he described Christianity as a threat to Empire — not yet at its heart but close to it, among Rome's equites and Senators. Christian apologists interpreted his eventual fate — a disgraceful capture and death — as divine judgement.

The next forty years were peaceful; the Christian church grew stronger and its literature and theology gained a higher social and intellectual profile, due in part to its own search for political toleration and theological coherence. Origen discussed theological issues with traditionalist elites in a common Neoplatonist frame of reference — he had written to Decius' predecessor Philip the Arab in similar vein — and Hippolytus recognised a "pagan" basis in Christian heresies. The Christian churches were disunited; Paul of Samosata, Bishop of Antioch was deposed by a synod of 268 both for his doctrines, and for his unworthy, indulgent, elite lifestyle. Meanwhile, Aurelian (270-75) appealed for harmony among his soldiers (concordia militum), stabilised the Empire and its borders and successfully established an official, Hellenic form of unitary cult to the Palmyrene Sol Invictus in Rome's Campus Martius.

In 295, Maximilian of Tebessa refused military service; in 298 Marcellus renounced his military oath. Both were executed for treason; both were Christians. At some time around 302, a report of ominous haruspicy in Diocletian's domus and a subsequent (but undated) dictat of placatory sacrifice by the entire military triggered a series of edicts against Christianity. The first (303 AD) “ordered the destruction of church buildings and Christian texts, forbade services to be held, degraded officials who were Christians, re-enslaved imperial freedmen who were Christians, and reduced the legal rights of all Christians... [Physical] or capital punishments were not imposed on them" but soon after, several Christians suspected of attempted arson in the palace were executed. The second edict threatened Christian priests with imprisonment and the third offered them freedom if they performed sacrifice. An edict of 304 enjoined universal sacrifice to traditional gods, in terms that recall the Decian edict.

In some cases and in some places the edicts were strictly enforced: some Christians resisted and were imprisoned or martyred. Others complied. Some local communities were not only pre-dominantly Christian, but powerful and influential; and some provincial authorities were lenient, notably the Caesar in Gaul, Constantius Chlorus, the father of Constantine I. Diocletian's successor Galerius maintained anti-Christian policy until his deathbed revocation in 311, when he asked Christians to pray for him. "This meant an official recognition of their importance in the religious world of the Roman empire, although one of the tetrarchs, Maximinus Daia, still oppressed Christians in his part of the empire up to 313.”