Leo the Great, as he is called, was a significant contributor to the centralization of the spiritual authority of the Church. He is one of the most well known of the popes of the early Church.
Leo, the son of Quintanus, was an Italian aristocrat and a native of Tuscany. He was born around 400. Apparently, he realized early on that he wanted to serve the Church, for, by the time of Pope Celestine I, who reigned 422-432, he was already a well known deacon. We know this because Cassianus of Gaul, who wrote "De Incarnatione Domini contra Nestorium" about 430, dedicated the work to Leo, whom, he said, had suggested that Cassianus write it.
It was also about this time that Cyril of Alexandria, in his major argument against Juvenal of Jerusalem, wrote to Rome asking for assistance. Leo appears to have been the one to help.
Shortly afterwards, Leo was sent to Gaul by the Emperor Valentinian III to settle a dispute and bring about a reconciliation between Aëtius, the chief military commander of the province, and the chief magistrate, Albinus. For an emperor to send out a deacon as his emissary implies that the deacon was not only holy, as needs be in the Church, but incredibly clever and with a good command of logic and debate.
In August of 440, Pope Sixtus III died while Deacon Leo was still in Gaul. He was chosen successor in abstentia and it took a month for him to return to claim his place on the throne of Peter whereupon he was quickly consecrated.
The aim of Pope Leo was to unify the Church, which had been wracked with heresies for the past two hundred years. This was a very difficult achievement since there were many obstacles in his way. The Western Empire was disintegrating and the Eastern Church was experiencing most of the dogmatic controversies.
First, Leo heard that the Pelegians were being received back into the Church without renouncing their heretical beliefs. This he had to immediately stop by holding a provincial synod, calling these people to confess their heresies.
Then, the Manicheans from Africa had been thrown out and had all arrived in Rome, secretly practicing their religion. In 443, Leo asked all bishops, priests and the faithful to report on the Manicheans. Within a year, most had converted to the orthodox faith or had left Italy. At Leo's encouragement, Emperor Valentinian III published an edict listing seven punishments for any Manichean. Most of them left the provinces, only to receive the same treatment in the Eastern Church.
Being very concerned about Church unity, Leo had extreme run ins with both Hilary, bishop of Arles as well as vicar of eastern Gaul, and the diocese of Thessalonica. Leo insisted that all concerns of succession and complainst of bishops should go through the office of the bishop of Rome, not through the vicars. In the end, his arguments held.
One thing that Leo is perhaps most known for is his negotiation with Attila the Hun. The Huns had swept across the empire and had destroyed much. Then, in 452, they entered Italy. The emperor sent Leo and two others to negotiate with Attila, the leader. It is not known what was said, but Attila and his men turned around and left. It may have been the outbreak of the plague, a lack of food or Leo's persuasive nature. We will never know.
However, three short years later, Genseric and his Vandals entered Rome and did damage for two weeks before Leo persuaded him, too, to cease.
Leo was involved in temporal activity also. He repaired the roof of St. Paul Outside the Walls, which had been damaged by lightning. He persuaded the empress to commission the mosaic of the Arch of Triumph, which is still erect. He restored St. Peter's on the Vatican Hill. And he allowed the building of the Church of St. Stephen on the Via Appia, whose ruins have been found.
While doing all this, Pope Leo wrote extensively and delivered hundreds of sermons which are very succinct and clear, in order to explain in detail ways and modes of spirituality and holiness. Over one hundred are extant today. His legacy is mostly concerned with theological questions concerning the person of Jesus Christ (Christology) and his role as mediator and savior (Soteriology). this is partially connected to the Council of Chalcedon in which Roman legates participated in Leo's name. For all these activities and writings, Pope St. Leo has been declared a Doctor of the Church.
Pope St. Leo said: "Virtue is nothing without the trial of temptation, for there is no conflict without an enemy, no victory without strife."