Today is the memorial of Pope St. Leo the Great. Not only is he recognized as one of the great popes, he is also one of the Doctors of the Church. Leo was bishop of Rome from 440 to 461, and he is responsible for defining the papacy as we've come to know it. Not only did Leo provide much of the theological basis for papal authority, he exercised that authority in governing the Church, resolving disputes between contentious bishops, and establishing the principle that the Church is separate from the state.
Leo famously rode out to meet Atilla the Hun, convincing him not to sack and pillage Rome. He was not so successful with the Vandals, who did enter Rome, but pillaged in a more gentle manner than was their typical mode.
Leo forcefully combatted Christological heresies, such as Nestorianism (that Christ was two persons, one human and the other divine) and Monophysetism (that Christ had only a divine nature and not a human one). He called the Council of Chalcedon to affirm that Jesus Christ was one person with two natures, fully God and fully man. Leo also did all that he could to make life miserable for the non-Christian Manicheans.
It is because of Pope St. Leo the Great that we see the bishop of Rome today as the successor of Peter, the vicar of Christ and the human head of the Church on earth.
Pope St. Leo the Great, pray for us!