Holding office in the late fifth century, Pope Leo I was the first to be called the “Great”—and for good reason. Pope Leo—whose feast day was celebrated last weekend—took on just about every major heresy of his time, established the dogmas of Christ as being fully man and fully God, asserted the primary of the papacy, and staved off a barbarian invasion of Rome.
The Heresy Killer: As Pope, Leo tackled both Pelagianism and Manichaeanism, a form of gnosticism. Soon after taking office, he banned the practice of admitted Pelagians to Communion without first requiring them to renounce their heresy. He was even more aggressive against the Manichaeans, according to the Catholic Encyclopedia: “This zealous pastor waged war even more strenuously against Manichæism, inasmuch as its adherents, who had been driven from Africa by the Vandals, had settled in Rome, and had succeeded in establishing a secret Manichæan community there. The pope ordered the faithful to point out these heretics to the priests, and in 443, together with the senators and presbyters, conducted in person an investigation, in the course of which the leaders of the community were examined.”
Dogma of Christ’s Two Natures: The heresy which Leo is perhaps most remembered for suppressing is Monophysitism, which held that Christ had one nature—presumably some mystical mixture of humanity and divinity. Leo is credited with orchestrating the dogmatic definition of Christ’s dual yet distinct natures that was issued from the Council of Chalcedon in 451 AD. (His views are spelled out in the Tome of Leo, see below.)
Primacy of the Pope: Leo is credited with asserting the authority and primacy of the successor to St. Peter in all Church matters, even enlisting the help of the current Roman emperor, Valentinian III, who issued an edict to the bishop in Gaul mandating “that nothing should be done in Gaul contrary to ancient usage, without the authority of the bishop of Rome, and that the decree of the apostolic see should henceforth be law.” When St. Leo’s position on Christ’s natures prevailed at the Council of Chalcedon, it helped launch the papacy into the position of dominance it has enjoyed ever since.
Saving Rome from the Barbarians: In 452 AD, as the armies of Attila the Hun menaced Rome, Leo rushed out to meet him in northern Italy, in what surely must have been an epic encounter between two different worlds. The meeting ended with Attila agreeing to not invade Rome. Some historians have questioned just how much credit should go to Leo for this. Some say the real reason Attila withdrew was an outbreak of a plague and food shortages in their camps. Others have speculated that Leo gave Attila “a large sum of gold” in return for sparing Rome. One thing is undisputable though: Attila retreated after his meeting with the Pope. Leo was unable to save the city a second time from barbarian hordes just a few years later, when the Vandals sacked the city. But he reportedly was able convince the Vandals from killing Roman citizens and burning city buildings. “These incidents show the high moral authority enjoyed by the pope, manifested even in temporal affairs,” the Catholic Encyclopedia concludes.
Doctor of the Church: Leo is honored as a doctor of the Church and his writings—most famously the Tome of Leo—alone likely would earn him the epithet of “Great.” As one of his biographers wrote: “The writings of this great pastor are the monuments of his extraordinary genius and piety. His thoughts are true, bright, and strong; and in every sentiment and expression we find a loftiness which raises our admiration. By it we are dazzled and surprised in every period, and whilst we think it impossible that the style should not sink, we are astonished always to find it swelling in the same tenour, and with equal dignity and strength.”