Roman religion can refer to several different concepts. The earliest concept that we might refer to as "Roman religion" was the religion of the early Roman people, which evolved into the Roman Pantheon and the Imperial Cult, practices that co-existed with the wide variety of religious practice found in all parts of the Roman Empire. Finally, many of these religious practices were swept away by Christianity.
The foundation of Roman religion can be found in the religion of the early Roman people, which is in some respects an animistic one. In this ancient Roman religion, every village, every separate part of a village, had its own local spirits. Spirits were everywhere. There were no formal rituals. As time progressed, anonymous spirits that acquired a shared following might acquire names and specific rituals. There were spirits of the sky (Jupiter), the door (Janus), the hearth (Vesta), boundary stones (Termini), hinges (Cardea), grain (Ceres), planting (Saturn), the forest (Mars), vines and gardens (Venus), mildew (Robigus), flowers (Flora), fruits (Pomona), wine (Bacchus), the stored harvest (Consus), and so on and so forth. These spirits were not (yet, anyway) developed, anthropomorphic gods. It was a down-to-earth, organic religion of the everyday, of the home, of the farm, of the pasture. While there was no particular focus on morality, it did encourage duty, discipline, hard work, loyalty, and courage.
Early Romans thought of places, things, people, and gods as being "numinous," possessing a sort of invisible force. Later Roman writers would refer to this force as numen (plural numina). The concept of numen was both particular and general; a spirit is a numen and its life force is numen in general. A numen was viewed as having a consciousness and supernatural powers that could be used for good and evil. These numina permeated the world, and nothing happened except through these numina. Much of early Roman religious practices involved attempts to please these spirits and gain their favour. Some spirits were considered hostile and had to be bribed by the appropriate offerings, prayers, and the like. Other spirits were considered friendly, and their favour could be secured with the appropriate offerings, prayers, and the like. This religion was generally concerned with the day-to-day needs of the present, not with future concerns.
This old Roman religion of the house and field was a vital, organic one; subsequent waves of religious change did not completely obliterate these practices (it is not coincidental that the word "pagan" comes from the Latin pagani, which refers to the people of the country villages). It was the last non-Christian religion to be exterminated by Christianity in the West. Upon investigation, the survival of various pagan rites can be found even today. Certainly many of the customs of Christmas, such as the giving of gifts, descend from the Roman winter festival of the Saturnalia. The similarity between the cult of some saints and ancient pagan practices is in many cases more than coincidental. Some practices of the modern-day country-dwellers in southern Europe, such as some of the strange rites found at Feste in some of the small Italian towns, can still be traced to these ancient practices.
As Rome matured, two forces came into play to change Roman religion. The first was the consolidation of many communities in Italy under a strong central government in Rome, and the second was contact with the Greeks, both through trade and through those living in Southern Italy. These forces would change Roman religion into a more institutional religion, one centred in Rome, promoting the good of the Roman state.
As the new city-state of Rome came into conflict with the other Italian people, the task of assimilating the conquered population started with assimilating their religion too. At this time many of the deities of the Italian tribes were introduced into Rome. For example, the goddess Minerva likely came from the Etruscan Falerii, and Castor and Pollux came from Tusculum after the battle of Lake Regillus in 496 B.C.
From contact with the Greek world, Rome received its heritage of religion, among other things. Rome would eventually imitate the Greek pantheon, adapting it to its own forms. Jupiter would become Zeus, Juno Zeus' wife Hera, Mars Ares, Minerva Athena, Bacchus Liber, and so on to even obscure deities. The Roman gods would even acquire the outward representation of their Greek counterparts, their personal characteristics, their family relationships, and their legends.
The original Roman religion, while it might be considered "primitive" by some, could be a true spiritual conception of the universe and of human life, while this Greco-Roman religion did not have that down-to-earth, organic style. It tended to be rather more formal and sterile (and, as it turns out, in contrast with the old Roman religion, this Greco-Roman religion left us no legacy, literature excepted). This sterility led many educated people to question, at least privately if not publicly, whether these gods really existed or not. For those whose spirituality could not be satisfied with the Roman pantheon, there were many alternatives, though.
One alternative was that of various Greek philosophical systems, primarily in the philosophical position known as Stoicism. Stoicism was not just a set of abstract ideas, but a way of life for both the humble and uninstructed and for the lofty and learned. Stoicism does emphasise various virtues; writers such as Epictetus and Marcus Aurelius have emphasised fortitude, indifference to pain and sorrow, and more. Gods were seen not as the capricious beings of mythology, but as being immortal, in charge of the universe, and benevolent towards and seeking the good of humanity. By the early Empire, as shown in the writings of Seneca, the idea of gods had turned into an idea of one God (usually referred to as Jupiter), with the other gods being regarded as forms of Jupiter or as personifications of forces of nature.
Another Greek philosophy was that of Epicureanism. Epicurus wanted to free men from what he described as the myths about the gods, because he regarded them as contradictory to the ideal of tranquility of mind. His disciple Lucretius took this idea and ran with it: The gods are actually the ideal of tranquility; they cannot be disturbed by events on Earth; they don't care at all about the affairs of humanity and won't interfere; they neither pay attention to prayers and sacrifices nor vent their wrath in the absence thereof. The gods did not create, nor do they govern, the world; the world was created and is governed by natural law. Yet, even though Lucretius held these positions, he was not an atheist. The gods still exist, and people can apprehend them, thus bringing them closer to the ideal of tranquility. Epicureanism should not be confused with the way that the words "epicurean" and "epicure" are used today; while Epicurus believed pleasure to be the greatest good, he believed that the way to attain pleasure was through living modestly and acquiring knowledge, not through indulging in food, drink, parties, sex, and so on.
Still others sought the vital, emotional forms of worship found in the Eastern fringes of the Empire. As early as 204 B.C., the cult of the Magna Mater of Phrygia had been brought to Rome, and her strange orgiastic ritual won such a popular influence that the senate was forced to regulate it. Later on, from Cappadocia, came the worship of Mâ, with its initiation rite of baptism in bull's blood, and, from Egypt, came the cult of Isis with its fasts and festivals, at which was represented the resurrection-drama of Osiris. These were quite popular and threatened to supercede the Greco-Roman religion. Augustus and other Emperors tried to rejuvenate the Greco-Roman religion, yet these mystery-cults continued to gain acceptance.
One of the greatest of these cults was the Persian cult of Mithras. It had originally been brought home by Sulla's soldiers, but caught on slowly; however, by the third century Mithras was the most popular god among the soldiers, who carried his worship to every corner of the Empire. There were many similarities, at least superficially, between Mithraism and Christianity. Both were heresies of older monotheistic religions, Christianity of Judaism and Mithraism of Zoroastrianism. Both Jesus and Mithras were born on December 25 and were a link between God and man. Both had become incredibly popular by the third century. Were it not for one fatal weakness, it is possible that Mithraism, not Christianity, could have become the religion of the Empire: Mithraism was not open to women.
The Roman Empire was flexible when it came to religion. It was tolerant of virtually all religions, provided that adherents were willing to make a token sacrifice to the Imperial cult from time to time. Almost all faiths were compatible with this idea. A very few were not. One of these few that consistently gained a following in the Roman Empire was Christianity. The Roman Empire as a whole didn't really know what to make of this religion that did not tolerate any of the other gods of the Empire. It has been suggested that pagan Romans would have gladly welcomed Jesus to the Roman pantheon, but Christians would likely have been horrified by that idea.
Persecutions of Christians in the Roman Empire are well-documented; however, the scope of most persecutions was greatly exaggerated by later Christian writers. While a cry of "Christians to the lions" might often have been heard when a crisis befell part of the Empire, actual persecutions were uncommon and merely of a local nature until the middle of the third century. While the well-educated Emperors of the second century may have thought highly negatively about Christianity, they did not seriously think that it was responsible for floods, earthquakes, crop failures, pestilence, "barbarian" invasion, or whatever problems might be befalling the Empire at that moment. However, in the third century it became more common for a less-educated class of man to become Emperor, and such a person might be more likely to think so. The first Empire-wide persecution took place around 250 A.D., during the reign of Decius, and many Christians were put to death. After a stop to these persecutions about a decade afterwards, they were revived by Diocletian at the start of the fourth century.
These persecutions didn't really discourage adherents of Christianity; Christians, or at least the more faithful ones, felt martyrdom to be a privilege, the most desirable end, and the sight of these martyrs undergoing the tortures that they were put through demonstrated to pagans the strength of this new religion, thus creating new converts. Persecution aside, people were attracted to Christianity for several reasons. Perhaps the strongest attraction is that, unlike most pagan religions, Christianity promised paradise in the world to come—a big deal in the tumultuous third century, when things in the present world were no longer going well for many people. By the time of Diocletian, Christians may have made up been one-fifth and one-third of the entire population of the Empire and might even have been a majority in the eastern part of the Empire, although exact numbers are impossible to obtain. By this point, Christianity was already so big that even Diocletian's Empire-wide persecution could not rid the Empire of Christanity.
Shortly after the persecution of Diocletian, Christianity gained ascendency throughout the Empire, during the reign of the emperor Constantine. Constantine's patron god was Sol Invictus, the god of the unconquered sun. According to tradition, Constantine embraced Christianity after seeing his victory at the Battle of the Milvian Bridge as being the work of the Christian God. The reality is much more complex. It seems to be the case that Constantine was pagan in 306 and Christian at the time of his death in 337, but it is not 100% clear how and at what time his beliefs evolved in the intervening years. Following Constantine, almost all subsequent emperors were Christian. The sole exception was Julian II, emperor from 361 to 363, who renounced Christianity and devoted his life to re-energizing the pagan religions of the Empire. However, his life was much too short for his work to have any long-term influence. It is interesting to speculate on what the results to history would have been had he lived several decades longer.
With Christianity having gained the upper hand in the Empire, Christians began suppressing all of the other religions of the Empire. This culminated with the outlawing of pagan worship in 391 by Emperor Theodosius. Christians acted as if getting pagans to accept Christianity was a matter of life and death, and, to Christians, it really was—a matter of eternal life and death, in fact.
Another matter of eternal life and death for early Christians was that of heresy. The question of correct doctrine is one that had been debated since the earliest days of Christianity, even at the time of the writing of the New Testament, as various books evince. The Acts of the Apostles and Paul's letter to the Galatians recount friction between Jewish Christians and Gentile Christians; the Book of Revelations provides a warning against the heresy of the Nicolaitans (although it is not known today exactly what heresy this group professed); the First Letter of Paul to Timothy, which most scholars believe to post-date Paul, contains warnings against gnostic heresies. In the fourth century, as Christianity became the dominant religion in the Empire, it became important to those in power that everyone be practicing the same version of Christianity, and heresies were suppressed, sometimes brutally. This drive would climax in the sixth century, when Emperor Justinian attempted, sometimes violently, to obtain divine favour for his rule by uniting the entire Empire under Orthodox Christianity. Unfortunately for the Empire, the main result of this action was that, in the seventh century, areas of the Empire where other forms of Christianity thrived welcomed the Muslim invaders upon their arrival.
While Christianity stood aloof from Greco-Roman culture, it borrowed freely from it, even in the earliest days of the religion; for example, St. Paul was from Tarsus, a Stoic stronghold, and his epistles exhibit Stoic influence. Over the centuries, further synthesis of Christian and pagan practices took place. A large number of saints and martyrs had sprung up to take over the competing functions of other gods. For example, sleeping in a church dedicated to the twin Saints Cosmas and Damian could produce the same sort of cures that could previously be obtained by sleeping in a temple of the twin gods Castor and Pollux. Many other similarities, such as the celebration of Christmas, have already been noted. The influence of Rome upon Christianity was also felt in more subtle ways. It is inconceivable that Christianity could have spread the way it did without the Empire enabling easy travel by road or sea. Another significant gift of Rome to Christianity is the idea of a hierarchical organization, as still found in the Catholic Church today. The idea of a single universal church is also primarily due to the influence of a universal Roman Empire. The head of the Roman Catholic Church, the pope, uses the title pontifex maximus, the title formerly held by the emperors of Rome from Augustus to Gratian and, before Augustus, the pagan high priests of Rome. Finally, Christian architecture was significantly influenced by Roman architecture.
In the fifth century, the western half of the Roman Empire collapsed. It was a gradual process, culminating in the end of the line of Roman Emperors in the west, in 476. Many, both then and in subsequent centuries, placed much of the blame at the feet of Christianity, for encouraging people to think about the world to come at the expense of the present one, encouraging pacifism, and causing able-bodied men to retire to monasteries instead of joining the army, serving the state, or pursuing other productive activities. However, Christianity alone cannot tell the entire story of the collapse of the Empire. After all, many of the Germanic tribes had embraced Christianity, if Arian Christianity, by this time, and they were doing alright. The Eastern half of the Empire, where Christianity was more prevalent than in the Western half, also survived the fifth century intact. It seems likely that chronic economic and political problems and external pressures played a greater role in the collapse of the Western Empire than did Christianity. Yet, while the Empire collapsed, its religion survived, and is one of the major legacies of the Roman Empire to the world of today.