In ancient Rome, religion was a combination of several cult practices and did not have a specific single belief system. Gods, who were referred to as numina, were responsible for specific aspects of human activity. This set of beliefs was based heavily on Greek mythology.
The influence of Greek culture, religion especially, increased during the age of the Roman republic. In fact, many of the gods in Roman religion were the same gods as those in Greek mythologies, but with different names. For instance, Zeus, who was the chief Greek god, was referred to as Jupiter in the Roman religion.
As Rome evolved into an empire, religion evolved as well. The divinity of the emperor himself became an important part of religion in the Roman Empire. Augustus and many subsequent emperors were deified to further revere the emperors as gods.
As the Empire expanded, many people from various cultures were incorporated and many foreign cults were embraced and became popular, such as Egyptian worship of Isis and Persian worship of Mithras.
Although Christianity was forbidden, many missionaries traveled across the Empire winning converts. After the Great Fire of 64 AD, Nero accused the Christians and many were executed. Persecution against Christians was fierce in the Roman Empire.
In the 2nd century, Christianity was spreading and gaining some momentum. In fact, under Constantine I, Christianity became the state religion in 380 AD and all cults were prohibited by Theodosius I in 391 AD.
After it was legalized, Christianity organized much like the Empire itself. Geographic regions, called dioceses, also called “sees” were overseen by bishops. The most prestigious dioceses were Rome, Constantinople, Jerusalem, Antioch and Alexandria. They were prestigious based on the supposed founders. For instance, St. Peter was considered the founder of the See of Rome (although no history has proven that Saint Peter himself ever set foot in Rome until he was arrested and crucified).
The Pope, the Bishop of Rome carried primacy based on the fact that the See of Rome was paramount among all others. The reason for this is based on 3 ancient traditions: (1) Peter was preeminent among the apostles, (2) Peter ordained his successors, and (3) the bishops are the successors of the apostles. In reality, the prestige of the Bishop of Rome and the preeminence of the See of Rome were taken for granted and usually needed no explanation because Rome was essentially the capital of the known world.
When the last Roman Emperor, Flavius Romulus Augustus, abdicated the throne in 476, ending the Roman Empire, the Bishop of Rome dominated as the chief religion figure.