Before Constantine's conversion, Christianity had expanded through missionary work, which succeeded in founding congregations across the Empire but did not seriously disrupt polytheism or the Empire’s religious diversity. Imperial sponsorship changed that because it linked secular power to Christian identity. Following Constantine's conversion, being a Christian became a way to get ahead in the Roman power structure, and over time it became a liability to remain a polytheist. Thus, whereas early Christianity had been a religion of the common people, Roman elites flocked to convert after Constantine did so in order to stay in the emperor's good graces.
Early Christians had already developed a distinct hierarchy of worshipers, a divide between priests and worshipers. Bishops were the head of each city's congregation, and they oversaw a staff of priests and deacons who interacted with everyday worshipers and led services. The bishops of main cities, usual the imperial capitals of their respective provinces, came to be called an archbishop. Each bishop oversaw activity in the diocese, again following the imperial structure, in instructing people in Christian doctrine and in building charity networks. One important effect was that the church actively supported charities for the poor and hungry, a practice which won over new converts. This marks one of the first times in history when a religion linked together a message of compassion for the needy and real, practical efforts to help the needy. In another strong contrast with Roman practice, Christianity saw disenfranchised groups like women and the poor (not to mention poor women) play major roles in the church’s organization, especially before “official” Christianity came into being under Constantine.
Almost immediately after Constantine became a Christian, bishops saw their secular power increase dramatically. He allowed bishops to serve as official judges, giving Christians the ability to request a bishop instead of a non-Christian judge in trial. Bishops also moved in administrative circles, representing not just the church but their cities in actions and requests before governors and assemblies. In short, bishops suddenly assumed power on par with that of the traditional Roman nobility, directly linking power within the Christian church hierarchy to power within the Roman political system.
The most important bishop was the archbishop of Rome, who for the first few centuries of Christianity was just one among several major church leaders. Originally, the archbishops of cities like Alexandria and Damascus were of comparable importance to the Roman archbishop, but over time Roman archbishops tried to assert authority over the entire church hierarchy in the west. Their authority, however, was not recognized in much of the eastern part of the Empire, and it should be emphasized that it took more than six centuries after Constantine for the Roman archbishop’s authority to receive acceptance even in the west. Eventually, however, that authority was at least nominally in place, and the Roman church leader came to be known as the "pope," meaning simply "father," of the church.
The pope's role as leader of the church emerged for a few reasons. First and foremost, the symbolic power of the city of Rome itself gave added weight to the Roman archbishop's authority. Second, there was a doctrinal tie to the Apostle Peter, who was supposed to have been given the symbolic keys to heaven directly from Christ, which were in turn passed on to his successor in Rome (the archbishop of Rome) before being crucified. Roman archbishops could thus argue that the Christian church itself was centered in Rome, and that they inherited the spiritual keys to heaven upon taking office - this concept was known as the “Petrine Succession.” By the mid-fifth century CE, the popes were claiming to have total authority over all other bishops, and at least some of those bishops (in Western Europe, at any rate) did look to Rome for guidance. In later centuries, the mere fact that the early popes had claimed that authority, and certain bishops had acknowledged it, was cited as “proof” that the Roman papacy had always been the supreme doctrinal power in the Church as a whole.