The Romans, according to the orator and politician Cicero, excelled all other peoples in the unique wisdom that made them realize that everything is subordinate to the rule and direction of the gods. Yet Roman religion was based not on divine grace but instead on mutual trust (fides) between god and man. The object of Roman religion was to secure the cooperation, benevolence, and “peace” of the gods (pax deorum). The Romans believed that this divine help would make it possible for them to master the unknown forces around them that inspired awe and anxiety (religio), and thus they would be able to live successfully. Consequently, there arose a body of rules, the jus divinum (“divine law”), ordaining what had to be done or avoided.

These precepts for many centuries contained scarcely any moral element; they consisted of directions for the correct performance of ritual. Roman religion laid almost exclusive emphasis on cult acts, endowing them with all the sanctity of patriotic tradition. Roman ceremonial was so obsessively meticulous and conservative that, if the various partisan accretions that grew upon it throughout the years can be eliminated, remnants of very early thought can be detected near the surface.

This demonstrates one of the many differences between Roman religion and Greek religion, in which such remnants tend to be deeply concealed. The Greeks, when they first began to document themselves, had already gone quite a long way toward sophisticated, abstract, and sometimes daring conceptions of divinity and its relation to man. But the orderly, legalistic, and relatively inarticulate Romans never quite gave up their old practices. Moreover, until the vivid pictorial imagination of the Greeks began to influence them, they lacked the Greek taste for seeing their deities in personalized human form and endowing them with mythology (myth). In a sense, there is no Roman mythology, or scarcely any. Although discoveries in the 20th century, notably in the ancient region of Etruria (between the Tiber and Arno rivers, west and south of the Apennines), confirm that Italians were not entirely unmythological, their mythology is sparse. What is found at Rome is chiefly only a pseudomythology (which, in due course, clothed their own nationalistic or family legends in mythical dress borrowed from the Greeks). Nor did Roman religion have a creed; provided that a Roman performed the right religious actions, he was free to think what he liked about the gods. And, having no creed, he usually deprecated emotion as out of place in acts of worship.

In spite, however, of the antique features not far from the surface, it is difficult to reconstruct the history and evolution of Roman religion. The principal literary sources, antiquarians such as the 1st-century-BC Roman scholars Varro (Varro, Marcus Terentius) and Verrius Flaccus (Verrius Flaccus, Marcus), and the poets who were their contemporaries (under the late Republic and Augustus), wrote 700 and 800 years after the beginnings of Rome. They wrote at a time when the introduction of Greek methods and myths had made erroneous (and flattering) interpretations of the distant Roman past unavoidable. In order to supplement such conjectures or facts as they may provide, scholars rely on surviving copies of the religious calendar and on other inscriptions. There is also a rich, though frequently cryptic, treasure-house of material in coins and medallions and in works of art.

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 (animism), 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).

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!”).

The apparent amalgamation of the Latin and Sabine villages of Rome coincided with, or more probably was soon followed by, a period in which Rome was under the control of at least one dynasty (the Tarquins) from Etruria, north of the Tiber (c. 575–510 BC, though some scholars would extend this domination to c. 450).

The Etruscans felt profound religious anxieties and were more devoted to ritual than any other people of the ancient Western world. Though sources are, again, late and unsatisfactory, it appears that they possessed a comprehensive collection of rules regulating these rites. Etruscan culture was heavily based on influences from Greece in its orientalizing period, conveyed mainly through Greek centres (such as Cumae) in Campania, colonized by Euboeans, who were also prominent in Syrian markets. But the religion of Etruria proclaims a very un-Greek view of the abasement and nonentity of man before the gods and their will.

The Roman religion continued to display certain obvious debts to the period when the city had been under Etruscan control. It is true that the Roman shades (Di Manes) were much less substantial than the fantastic Etruscan conceptions and, although Etruscan divination by the liver and entrails survived and later became increasingly fashionable in Rome, Roman diviners in general, products of a more realistic and prosaic society, never aspired to such precise information about the future as the Etruscans had hoped to gain. Yet, it was the Etruscans who first gave a vigorous definition to Italian religious forms. Indeed, many of the religious features that patriotic historians preferred to ascribe to the mythical King Numa Pompilius (who was supposed to have been Romulus' Sabine successor in the 8th century BC—the man of peace following the man of war) date, in fact, from the period of Etruscan domination two centuries later. Nevertheless, Romans acknowledged a debt to Etruria that included much ceremony and ritual and the plan, appearance, and decoration of a number of temples, notably the great shrine of the Capitoline Triad, Jupiter, Juno, and Minerva. The Romans also were indebted to the Etruscans for their first statues of gods, including the cult image of Jupiter commissioned from an Etruscan for the Capitoline temple. Such statuary, showing the gods in human shape, encouraged the Romans to think of their gods in this way, with the consequent possibility of investing them with myths, which thereafter gradually accumulated around them in the form of Hellenic stories often infused with a native patriotic element.

The festivals (feast) it records, of which the earliest are indicated in large letters, reflect a period of transition between country and town life. Though local cult continued to remain active, many forms of worship hitherto maintained by families and farms had now been taken over by the comparatively mature Roman state. The state management blocked any tendency toward spiritualization and removed the need for any vigorous individual participation; however, by ensuring that the gods were conciliated by a schedule corresponding to the regular process of nature, it made the individual citizens feel for centuries that relations with the supernatural were being maintained safely.

Even if, as tradition records, a coup d'état dislodged the Etruscan kings before 500 BC, in the first half of the 5th century there was no weakening of trade relations with Etruria. Its southern cities, such as Caere (Cerveteri) and Veii close to Rome, had long used the Greek city of Cumae as a commercial outlet, converting it into an important grain supplier. And now Rome, faced with a shortage of grain, arranged for it to be imported from Cumae. The same city also influenced the foundation of Roman temples in the Greek (Greek religion) style. Rome, which had already become accustomed to Greek religious customs in the Etruscan epoch, now showed a willingness to absorb them. This forms a strange contrast to its deeply ingrained religious conservatism. Moreover, at some quite early stage (though there is no positive evidence of the practice until the 3rd century), Romans borrowed from elsewhere in Italy a special ritual (evocatio) for inviting the patron deities of captured towns to abandon their homes and migrate to Rome.

In an emergency in 399 BC, during a difficult siege of Veii, Rome carried Hellenization further by importing a Greek rite in which, as an appeal to emotional feeling, images of pairs of gods were exhibited on couches before tables spread with food and drink; this rite ( lectisternium) was designed to make them Rome's welcome guests. From the same century onward, if not earlier, pestilences were averted by another ritual ( supplicatio), in which the whole populace went around the temples and prostrated themselves in Greek fashion. Later the custom was extended to the celebration of victories.

Astrological practices received encouragement from Stoic philosophy, which was introduced to Rome in the 2nd and early 1st centuries BC, notably by Panaetius and Poseidonius. The Stoics saw this pseudoscience as proof of the Platonic unity of the universe. Stoicism affected Roman religious thinking in at least three other ways. First, it had a deterministic effect, encouraging a widespread belief in Fate and also, somewhat illogically, in Fortune (Fortuna), both of which were revered in other parts of the Mediterranean and Middle Eastern world. Second, Stoicism infused a new spirituality into religious thinking by its insistence that the human soul is part of the universal spirit and shares its divinity. Third, the moral implication of this, as the Stoics pointed out, was that all men are brothers and must treat each other accordingly. This demonstration struck a chord in the psychology of the Romans, who possessed strongly ethical inclinations and now, at last, saw this trend supported and justified by a philosophical sanction that their formalistic religion had not provided. In changing times of imperialism, materialism, and widespread heart-searching, the state religion had failed to fill the vacuum, and philosophy stepped in instead. At the same time the negative approach of Roman religion to the afterlife was counteracted by an influx of speculations that blended theology, mysticism, and magic and claimed the mythical Orpheus and the part historical, part legendary Pythagoras as prophets.

While their national poet Ennius (Ennius, Quintus) helped to diffuse such beliefs, he and the comic dramatist Plautus ridiculed the traditional Roman gods on the stage. The upper-class attitude of the times was expressed by the historian Polybius, the priestly lawyer Scaevola, the scholarly Varro, and the orator and philosopher Cicero (Cicero, Marcus Tullius), who maintained that the importance of religion was political, residing in its power to keep the multitude under control, to prevent social chaos, and to promote patriotic feeling.

Octavian himself took the name Augustus, a term indicating a claim to reverence. This did not make him a god in his lifetime, but, combined with the insertion of his numen and his genius (originally the procreative power that enables a family to be carried on) into certain cults, it prepared the way for his posthumous deification, just as Caesar (Caesar, Julius) had been deified before him. Both were deified by the state because they seemed to have given Rome gifts worthy of a god. From earliest times in Greece there had been an idea that, if someone saved you, you should pay him the honours you would offer to a god. Alexander the Great and his successors had demanded reverence as divine saviours, and Ptolemy II Philadelphus of Egypt introduced a cult of his own living person. The Stoic belief that the human soul was part of the world soul was a corollary of the view that great men possessed a larger share of this divine element. Moreover, the 3rd-century-BC mythographer Euhemerus had elaborated a theory that the gods themselves had once been human; this idea was readily adapted to the supposed careers of Heracles (Hercules) and the Dioscuri (Castor and Polydeuces 【Pollux】); and the Romans applied it to their own gods Saturn and Quirinus, the latter identified with the national founder, Romulus, risen to heaven. And so it became customary—if emperors (emperor) (and empresses) were approved of in their lives—to raise them to divinity after their deaths. They were called divi, not dei like the Olympian gods; the latter were prayed to, but the former were regarded with veneration and gratitude.

As the empire proceeded and the old religion seemed more and more irrelevant to people's personal preoccupations and successive national emergencies, the cult of the divi, subsequently grouped together in a single Hall of Fame, remained foremost among the patriotic cults that were increasingly encouraged as unifying forces. Concentrating on the protectors of the emperor and the nation, they included the worship of Rome herself, and of the genius of the Roman people; for the army a number of special military celebrations are recorded on the Calendar of Doura-Europus in Mesopotamia (Feriale Duranum, c. AD 225–27). As for the ruling emperors, they were more and more frequently treated as divine, with varying degrees of formality, and officially they often were compared with gods. As monotheistic (monotheism) tendencies grew, however, this custom led not so much to their identification with the gods as to the doctrine that they were the elect of the divine powers, who were defined as their companions (comites). In pursuance of this way of thinking, as official paganism approached its last days, the emperors Diocletian and Maximian took the names Jovius and Herculius, respectively, after their Companions and Patrons Jupiter and Hercules.

By now, however, the humanistic idea that men could become gods had ceased to have any plausibility. Plotinus and his Neoplatonism, the dominant philosophy of the pagan world from the mid-3rd century AD, had given powerful, mystical shape to the Platonic and Stoic conception that the universe is governed by a single force. On the other hand, the greatest religious figure of the century, the Iranian Mani, who had started to preach in Mesopotamia c. 240, dramatically preached the opposing dualistic (dualism) idea that the world is the creation not only of a good power but of an evil one as well. Mani's church, which alarmed Diocletian and for a time attracted the great Christian theologian St. Augustine, absorbed many of the innumerable cults of Gnostics (Gnosticism) who claimed special knowledge (gnōsis) by illumination and revelation and taught how people can purge the nonspiritual from within themselves and escape their earthly prison. More impressively, the cult of the Persian Mithra (Mithraism) blended the dualism of Mani with the emotional initiations of the mystery religions (corrected by a much sterner tone of moral endeavour) and became a strong link between the cult of the Sun (sun worship) (which appealed to contemporary monotheists) and the fashionable revulsion from the senses that was shortly to lead to Christian monasticism. Like Christianity, Mithraism had its sacraments; but the life of Mithra exercised a less far-reaching appeal than the life of Christ, and Mithra's cult excluded women.

Christianity, unique in its universal charity and unique also in its demand for a noble effort of faith in Jesus' blend of divinity and humanity, was the religion that prevailed in the Roman world. It satisfied the emperor Constantine's (Constantine I) impulsive need for divine support, and from AD 312 onward, by a complex and gradual process, it became the official religion of the empire.

For a time, coins and other monuments continued to link Christian doctrines with the worship of the Sun, to which Constantine had been addicted previously. But even when this phase came to an end, Roman paganism continued to exert other, permanent influences, great and small. The emperors passed on to the popes the title of chief priest, pontifex maximus. The saints, with their distribution of functions, often seemed to perpetuate the many numina of ancient tradition. The ecclesiastical calendar retains numerous remnants of pre-Christian festivals—notably Christmas, which blends elements including both the feast of the Saturnalia and the birthday of Mithra. But, most of all, the mainstream of Western Christianity owed ancient Rome the firm discipline that gave it stability and shape, combining insistence on established forms with the possibility of recognizing that novelties need not be excluded, since they were implicit from the start.

The early Romans, like other Italians, worshiped not only purely functional and local forces but also certain high gods. Chief among them was the sky god Jupiter, whose cult, at first limited to the communities around the Alban Hills, later gained Rome as an adherent. The Romans gave Jupiter his own priest (flamen), and the fact that there were two other senior flamines, devoted to Mars and Quirinus, confirms other indications that the cults of these three deities, envisaged perhaps in some sort of association, belonged to a very early stratum (though the theory of their correspondence to the three-class social division of the early Indo-European-speaking peoples is generally unacceptable). Mars, whose name may or may not be Indo-European, was a high god of many Italian peoples, as liturgical bronze tablets found at Iguvium (Gubbio), the Tabulae Iguvinae (c. 200–c. 80 BC), confirm, protecting them in war and defending their agriculture and animals against disease. Later, he was identified with the Greek god of war, Ares, and also was regarded as the father of Romulus. Mars Gradivus presided over the beginning of a war and Mars Quirinus over its end, but earlier Quirinus had apparently, as a separate deity, been the patron of the Quirinal village before its amalgamation with the Palatine; subsequently he was believed to have been the god that Romulus became when he ascended into heaven.

A number of gods were spoken of as possessing accompaniments, often in the feminine gender; e.g., Lua Saturni and Moles Martis. These attachments, sometimes spoken of as cult partners, were not the wives of the male divinities but rather expressed a special aspect of their power or will. A similar origin could be ascribed to the worship of divine powers representing “qualities.” Fides (“Faith” or “Loyalty”), for example, may at first have been an attribute or aspect of a Latin-Sabine god of oaths, Semo Sanctus Dius Fidius; and in the same way Victoria may come from Jupiter Victor. Some of these concepts were worshiped very early, such as Ops (“Plenty,” later associated with Saturn and equated with Hebe), and Juventas (who watched over the men of military age). The first of these qualities to receive a temple, as far as is known, is Concordia (367), in celebration of the end of civil strife. Salus (health or well-being) followed in c. 302, Victoria in c. 300, Pietas (dutifulness to family and gods, later exalted by Virgil as the whole basis of Roman religion) in 191. The Greeks, too, from the earliest days, had clothed such qualities in words; e.g., Shame, Peace, Justice, and Fortune. In the Hellenic world they had a wide variety of signification, ranging from full-fledged divinity to nothing more than abstractions. But in early Rome and Italy they were in no sense abstractions or allegories and were likewise not thought of as possessing the anthropomorphic shape that the term personification might imply. They were things, objects of worship, like many other functions that were venerated. They were external divine forces working upon humans and affecting them with the qualities that their names described. Later on, under philosophical (particularly Stoic) influences that flooded into ethically minded Rome, they duly took their place as moral concepts, the Virtues and Blessings which abounded for centuries and were depicted in human form on Roman coinage (coin) as part of the imperial propaganda.

Little or no contribution to cosmology was made in the Roman world, and the demonstration of Aristarchus of Samos (c. 270 BC) that the Earth revolves around the Sun received virtually no support. The complicated geocentric interpretation that held sway in Rome was summed up in Cicero's Dream of Scipio. It formed the basis for the concept of the solar system on which the popular pseudoscience of astrology was founded, the Sun being regarded as the centre of the concentric planetary spheres encircling the Earth—not the centre of the cosmos in the sense of Aristarchus but its heart. From the 5th century BC onward this solar god was identified with Apollo in his role as the supreme dispenser of agricultural wealth. Possessor of a sacred grove at Lavinium, Sol Indiges was regarded as one of the divine ancestors of Rome. During the last centuries before the Christian era, worship of the Sun (sun worship) spread throughout the Mediterranean world and formed the principal rallying point of paganism's last years. Closely associated with the sun cult was that of Mithra, the Sun's ally and agent who was elevated to partake of communion and the love feast as the god's companion. Sun worship was popular in the army, and particularly on the Danube. Aurelian, one of the great military emperors produced by that area in the 3rd century, built a magnificent temple of Sol Invictus (the “Unconquered Sun”) at Rome (274). Constantine the Great (Constantine I) declared the Sun his Comrade on empire-wide coinages and devoted himself to the cult until he adopted Christianity in its stead.

Precedence among Roman priests belonged to the rex sacrorum (“king of the sacred rites”), who, after the expulsion of the kings, took over the residue of their religious powers and duties that had not been assumed by the Republican officers of state. Nevertheless, the hold exercised by the rex sacrorum and his colleagues was weakened by the Law of the Twelve Tables (c. 451–450 BC), which displayed the secular arm exercising some control over sacral law. As late as c. 275 BC the religious calendar was still dated by the rex sacrorum but by this time he was already fading into the background.

Very early origins can also be attributed to some of the flamines (flamen), the priests of certain specific cults, and particularly to the three major flamines of Jupiter, Mars, and Quirinus. Jupiter's priest, the flamen dialis, was encompassed by an extraordinary series of taboos, some dating to the Bronze Age, which made it difficult to fill the office in historic times.

Except for the rex sacrorum and flamen dialis, whose duties were unusually professional and technical, almost all Roman priesthoods were held by men prominent in public life. The social distinction and political prestige carried by these part-time posts caused them to be keenly fought for.

There were four chief colleges, or boards, of priests: the pontifices (pontifex), augures, quindecimviri sacris faciundis, and epulones. Originally three, and finally 16 in number, the pontifices (whose name may recall antique tasks and magic rites in connection with bridges) had assumed control of the religious system by the 3rd century BC. The chief priest, the pontifex maximus (the head of the state clergy), was an elected official and not chosen from the existing pontifices. The augures, whose name may have been derived from the practice of magic in fertility rites and perhaps meant “increasers,” had the task of discovering whether or not the gods approved of an action. This they performed mainly by interpreting divine signs in the movements of birds (auspicia). Such divination (augury) was elevated, perhaps under Etruscan influence, into an indispensable preliminary to state acts, though the responsibility for the decision rested not with the priests but with the presiding state officials, who were said to “possess the auspices.” In private life too, even as late as Cicero and Horace in the 1st century BC, important courses of action were often preceded by consultation of the heavens. The Etruscan method of divining from the liver and entrails of animals (haruspicina) became popular in the Second Punic War, though its practitioners (who numbered 60 under the empire) never attained an official priesthood.

Of the other two major colleges, the quindecimviri (“Board of Fifteen,” who earlier had been 10 in number) sacris faciundis looked after foreign rites, and the members of the other body, the epulones, supervised religious feasts. There were also fetiales (fetial), priestly officials who were concerned with various aspects of international relationships, such as treaties and declarations of war. Also six Vestal Virgins, chosen as young girls from the old patrician families, tended the shrine and fire of Vesta and lived in the House of Vestals nearby, amid a formidable array of prehistoric taboos.

The Roman calendar, as introduced or modified in the period of the Etruscan kings, contained 58 regular festivals. These included 45 Feriae Publicae, celebrated on the same fixed day every year, as well as the Ides of each month, which were sacred to Jupiter, and the Kalends of March, which belonged to Mars. Famous examples of Feriae Publicae were the Lupercalia (February 15) and Saturnalia (December 17, later extended). There were also the Feriae Conceptivae, the dates of which were fixed each year by the proper authority, and which included the Feriae Latinae (“Latin Festival”) celebrated in the Alban Hills, usually at the end of April.

Although ancestors were meticulously revered, there was nothing resembling the comprehensive Etruscan attention to the dead (death rite). In spite of elaborate philosophizing by Cicero and Virgil about the possibility of some sort of survival of the soul (especially for the deserving), most Romans' ideas of the afterlife, unless they believed in the promises of the mystery religions, were vague. Such ideas often amounted to a cautious hope or fear that the spirit in some sense lived on, and this was sometimes combined with an anxiety that the ghosts of the dead, especially the young dead who bore the living a grudge, might return and cause harm. Graves and tombs were inviolable, protected by supernatural powers and by taboos. In the earliest days of Rome both cremation and inhumation were practiced simultaneously, but by the 2nd century BC the former had prevailed. Some 300 years later, however, there was a massive reversion to inhumation, probably because of an inarticulate revival of the feeling that the future welfare of the soul depended on comfortable repose of the body—a feeling that, as sarcophagi show, was fully shared by the adherents of the mystery cults, though, on the rational level, it contradicted their assurance of an afterlife in some spiritual sphere. The designs on these tombs reflect the soul's survival as a personal entity that has won its right to paradise.

Though Roman religion never produced a comprehensive code of conduct, its early rituals of house and farm engendered a feeling of duty and unity. Its idea of reciprocal understanding between man and god not only imparted the sense of security that Romans needed in order to achieve their successes but stimulated, by analogy, the concept of mutual obligations and binding agreements between one person and another. Except for rare aberrations, such as human sacrifice, Roman religion was unspoiled by orgiastic rites and savage practices. Moreover—unlike ancient philosophy—it was neither sectarian nor exclusive. It was a tolerant religion, and it would be difficult to think of any other whose adherents committed fewer crimes and atrocities in its name.