The Religio Romana constituted the main religion of the city in ancient times. However, a number of other religions and imported secrecy cults remained represented within its ever-expanding limitations, counting Judaism, whose presence in the city dates sponsor from the Roman Republic and was sometimes compulsorily confined to the Roman Ghetto, as well as Christianity. In spite of initial persecutions, by the early 4th century, Christianity had turn into so widespread that it was legalized in 313 by Emperor Constantine I, and later made executive religion of the Roman Empire in 380 by Emperor Theodosius I, allowing it to increase further and ultimately wholly replace the declining Religio Romana.
Rome became the most excellent Christian city based on the custom that Saint Peter and Saint Paul were martyred in the city throughout the 1st century, coupled with the city's political significance. The Bishop of Rome, later identified as the Pope, claimed dominance over all Bishops and consequently all Christians on the foundation that he is the successor of Saint Peter, upon whom Jesus built his Church; his status has been enhanced since 313 during contributions by Roman emperors and patricians, including the Lateran Palace and patriarchal basilicas, as well as the visibly growing influence of the Church over the failing civil regal authority. Papal authority has been exercised over the centuries with unreliable degrees of success, at period triggering divisions amongst Christians, until the present.