By the third century, the church at Rome enjoyed the highest prestige of any church in Christendom, but as late as the time of Ambrose the Roman bishop was only one among many bishops. The fourth-century added to his prestige. In the Donatist schism, Constantine turned to the Bishop of Rome to be the mediator. In the Christological controversy, the opinion of Rome was important, though not decisive. Bishop Siricius near the end of the fourth century assumed authority to dictate to a Spanish bishop who had written for advice and suggested that the Spaniard transmit the Roman decision to other bishops in Spain. This authority was recognized by Emperor Theodosius. In the fifth century, Valentinian III decreed the supremacy of Bishop Leo I, giving him the power to make law for the whole Western Church. The East, of course, would admit no claims of Rome to supremacy. Leo I may properly be called pope, a dignity once used by every local pastor. Leo based his authority on Peter’s headship of the Church, and vigorously enforced his claims.
Impact: By the middle of the fifth century the church in Rome had been established as the supreme authority in the West.