In public life, the rituals of prayer and sacrifice were led by official priests acting on behalf of the state. Most priests of state religion were male and held office for life, one notable exception being the Vestal Virgins, the priestesses of Vesta. Vestals served full-time, whereas male priests were engaged only part-time in their priestly duties and frequently had active political and military careers. A man who had held a major political office automatically became a member of the Senate, a body that was consulted on both political and religious matters. Thus Cicero (On His House 1.1) notes the wisdom of having the same men controlling the worship of the gods and the vital interests of the state. The tradition of taking the auspices further exemplifies the inextricable relationship between religion and politics: the gods' will had to be sought before any public business was initiated, be it political or military.

We have seen how the state attempted to control the infiltration and adoption of foreign cults. A monotheistic religion, however, could not be reconciled with traditional Roman religious practice, as the above excerpt from Tacitus implies. The first extant reference to the presence of Jews in Rome is to their expulsion from the city in 139 BCE because they were trying to pass on their sacred rites to the Romans (Valerius Maximus, Memorable Deeds and Words 1.3.3). The numbers involved, however, were probably quite small, whether permanent residents or transients.

The sources for the next several decades are scanty, but by the late republic there was a substantial Jewish community permanently resident in Rome. Most of these people had been brought to Italy as captives, probably after Pompey's conquest of Syria in the 60s BCE. Many had been freed and later became Roman citizens (Philo, Embassy to Gaius 155). The Roman authorities were consequently faced with the permanent presence of people whose ancestral customs and conventions were very different from their own.

Ruins of ancient temples, interspersed among the excavated remains of the ancient city, still bear witness that Rome was indeed a city filled with gods and a sense of religion. Prominent among the remains in the Roman Forum are the temples of Castor, Saturn, Vesta, and the deified emperor Antoninus Pius and his wife Faustina. Overlooking the Roman Forum is the Capitoline Hill, the site of the temple of Jupiter the Best and Greatest. In the area of the Forum Holitorium near the river Tiber, the modern tourist can see ancient columns incorporated into a side wall of the church of San Nicola in Carcere. In the Campus Martius area, the emperor Hadrian's reconstruction of the Pantheon stands in all its grandeur amid the hustle and bustle of contemporary Rome, still bearing the inscription commemorating its original builder, Marcus Agrippa, who was a close associate of the emperor Augustus. The building was consecrated as a Christian church in the seventh century CE, another example of continuity and change within religious traditions.

As these excerpts from Cicero indicate, tradition retrojected Rome's religious institutions back to her mythical past, attributing the institution of the auspices and divination to Rome's founder, Romulus, and the establishment of ritual to Numa, Romulus' successor as king. The poet Ovid (43 BCE–17 CE) tells how Romulus and his brother Remus took the auspices to decide which of them should be the founder of Rome.

The poet Catullus speaks of a man seeming to surpass the gods. The more prosaic Cicero has the famous orator Mucius Scaevola say that he has always thought of another orator, Licinius Crassus, as a god whenever Crassus speaks. Virgil goes further. Without naming Octavian, he calls the man who has brought peace (otium) a god – and then declares that Octavian will always be a god to whom he, Virgil, will pay cult with tender lambs from his folds, because he can compose verses now that his cattle are allowed to wander – a probable allusion to the restoration of Virgil's property that had been confiscated by the triumvirs (Eclogue 1.6–10). Poetic hyperbole this may be, but Virgil, unlike Catullus, did not feel it necessary to add any caveat such as “if it is right to say such a thing.”

In a Roman context, “family” extends beyond our usual concepts of nuclear and extended family to include household slaves and their offspring. The laws of the Twelve Tables deriving from the mid-fifth century BCE reveal the extent of patria potestas, the power of the paterfamilias (father of the family) who had the power of life or death over the entire household. As is apparent in the prologue to Plautus' Pot of Gold, it was the duty of the paterfamilias to tend the family cult, maintain the pax deorum, and so ensure the well-being of the whole household. Cato's prayer of purification for his farm (On Agriculture 141) included the welfare of his crops and trees, the protection and welfare of his shepherds and their flocks, and good health and strength for himself, his family (domus), and his household (familia). Also included in the duties of the paterfamilias was the conducting of rituals concerned with birth, marriage, and death. Because these rituals were performed within the family and were not a matter of public record, the surviving literary evidence is scanty.

This book offers an introduction to the religion of ancient Rome through the early second century CE, using a selection of different kinds of ancient testimony to portray Roman action and opinion in areas that they or we would call “religion” – literary texts, epigraphic and numismatic evidence, together with visual representations of surviving artifacts. No background in Roman history is assumed, but readers are urged to consult the revised third edition of the Oxford Classical Dictionary, ed. Simon Hornblower and Antony Spawforth (2003), which contains articles by the foremost scholars of Roman religion and history.

The commentary on the ancient testimony is organized thematically and more or less chronologically within each theme. Putting these various sources together, however, does not produce a picture of Roman religion at any specific time but rather a patchwork of disparate sources of different dates. “Consider the source” is a maxim that must be constantly kept in mind. The traditional date of the foundation of Rome is 753 BCE, but the Roman literary sources date no earlier than the late third century BCE. Many of the stories concerning early Roman religion derive from authors writing several centuries after the events they purport to describe. First we must ask: who is the author, in what genre is he writing, when did he live, and how far removed in time is he from the events he is describing?

Inscriptions and coins offer evidence that is more or less contemporaneous with the event described or commemorated.

Examination of the ancient testimony from the mid-republic through the first three centuries of the empire reveals many variants of magic and the occult, including sorcery, witchcraft, incantations for healing, binding spells, necromancy (attempted communication with the dead), the wearing of amulets as protection against evil and disease, the interpretation of dreams, and astrology. These practices were at and beyond the fringe of traditional state and family religion, and so were perceived as a threat to the established order.

Our word “magic” derives from magus, a loanword from Persian originally meaning a fire priest, that came into Latin via the Greek magos. Apuleius, the late-second-century CE author of Metamorphoses or The Golden Ass, who himself was accused of magic (mageia), defines a magus as “someone who, through the community of speech with the immortal gods, possesses an incredible power of spells (cantamina) for everything that he wishes to do” (Apuleius, Apology 26.6).

In both republic and empire there are instances of the official adoption of new non-Roman gods, but often after initial resistance and unsuccessful attempts at suppression. The literary sources indicate that new cults were adopted by the state in times of crisis, especially plague or disaster in war. However, since the original infiltration of a new cult leaves little or no trace in the literary or archaeological record, it is quite possible that the literary record of the official adoption of a new cult merely indicates that the state authorities were sanctioning an already prevalent practice. Once a cult had taken hold of the populace, it generally would have been expedient to adapt it by eliminating unacceptable practices, rather than attempt to suppress it.