The origins of the Roman pantheon began with the small farming community that made up the ancient village of Rome. The foundations of the mythology included nameless and faceless deities that lended support to the community while inhabiting all objects and living things. Numen, as the belief in a pantheistic inhabitation of all things is called, would later take root in more clearly defined system of gods, but early on this belief that everything was inhabited by numina was the prevalent system.
Even though the early Romans were not very concerned with the distinct personalities of each god within their pantheon, there was a rigid clarification of what each particular deity was responsible for. All aspects of life within Rome were guided not only by the pantheon of familiar names we are accustomed to, but to the household cult of the Dii Familiaris as well. With this belief set, every family or household was believed to be assigned a guardian spirit known as the Lar Familiaris (Lars). All family functions included these spiritual guardians in some form or another. Among these spirits that played a role in the spiritual life of Romans were Genii for men and junii for women.
Each of these individual deities stayed with a person for life and represented the creative force that determined gender and allowed individuals to grow, learn and behave morally within society. The Dii Familiaris were so ingrained within the household that several spirits were assigned to specific responsibilities within a home. Forculus protected the door, Limentinus the threshold, Cardea the hinges, and Vesta the hearth.
Most of the Roman gods and goddesses were a blend of several religious influences. Many of these were introduced via the Greek colonies of southern Italy and others had their roots in the Etruscan or Latin tribes of the region. In some cases the Etruscan or Latin names survived throughout the cultural existence of Rome, but many were adopted so completely that they maintained their names from other cultures. In the east, the Greek names remained the choice of the people and the major gods of the system therefore, were known by both.
The gods of the Roman pantheon began taking on the forms known today during the dynasty of the Etruscan kings in the 6th century BC. These gods, Jupiter (Zeus), Juno (Hera), and Minerva (Athena), were worshiped at the grand temple on the Capitoline Hill. As Rome's power grew and expanded throughout the known world, the Roman Empire came into contact with the cultures and religious beliefs of many cultures. The Romans, happy to absorb and assimilate any culture they encountered thereby reaping the benefits of both its wealth and religious influence, were a mosaic of belief systems.
Foreign gods and customs not only played major roles but were also given temples and priesthoods within Rome itself. The goddess Cybele, a Phoenician god was adopted during the Second Punic War to counteract any benefit that Hannibal may have gained. Even after his defeat, Cybele remained an integral part of the Roman system. Another very popular foreign god was the Persian god Mithra. Overwhelmingly supported in the Legions, this deity offered eternal salvation for the immortal soul and its popularity helped pave the way for the later Christian cult whose similarities made its adoption less difficult.
With the passing of the Roman Republic into that of an Imperial system, the nature of Roman religion expanded again to include the Emperors themselves. Julius Caesar, having claimed to be a direct descendent of Aeneas, the son of Venus, was among the first to deify himself in such a manner. At first, such a system of human divinity was largely rejected by the masses, but the popularity of Caesar helped pave the way for future leaders.
As the Imperial system gained hold, it was common practice for the Emperors to accept divine honors before their deaths. These living gods, in some cases, required sacrificial rituals as signs of loyalty and ingrained themselves with the older more traditional pagan gods. The requirement of a sacrifice to the emperor, as well as the forced belief in the complete pantheon became a significant source of conflict with early Christians. As Christians refused to worship the emperor as a god, persecution of the Christians and conflict with the cult was a constant source of strife.
Emperor worship would continue until late in the western Empire until the reign of Constantine. In the early 4th century AD, Constantine either converted to Christianity or made it an acceptable part of Roman religion, eliminating the emperor deification altogether. Later Emperors such as Julian attempted to revive the old ways, but the deeply rooted Mithraism, and Christian cults combined were firmly set within Roman society. By 392 AD, Emperor Theodosius I banned the practice of pagan religions in Rome altogether and Christianity was, without question, the official religion of the state.