Today Leo was consecrated bishop of Rome becoming the pope. His papacy was very significant, famous for his meeting with Attila the Hun and persuading him to discontinue his invasion of Italy, he also suppressed the Manichean heresy and strengthened the authority of the church. He had assumed the papacy at a time of increasing barbarian invasions and decreasing imperial authority in the West, and so was one of the first bishops of Rome to promote papal primacy based on succession from Peter the Apostle as a means of maintaining unity among the churches. He was the first Pope to be buried in St Peters, as he wanted to be close to the apostle, and was also the first Pope to be given the title the Great, and was declared a doctor of the Church.
Attila had invaded Italy in 452, sacking cities such as Aquileia and heading for Rome. Atilla, the leader of a tribal empire consisting of Huns, Ostrogoths and Bulgars, among others, in Central and Eastern Europe. was one of the most feared enemies of the Western and Eastern Roman Empires. He had already plundered the Balkans, but was unable to take Constantinople. His incursion into Western Europe would get as far as Orléans in France before he was stopped in the Battle of the Catalaunian Plains. After his incursion into Northern Italy, he allegedly demanded that the sister of the reigning Emperor Valentinian III be sent to him with a dowry. In response, the emperor sent three envoys to negotiate with Attila: including Pope Leo. According to the historian Prosper of Aquitaine who was alive at the time of the event, Attila was so impressed by Leo that he withdrew. Paul the Deacon, in the late 8th century, in a less historically reliable account, relates that an enormously huge man dressed in priestly robes and armed with a sword, visible only to Attila, threatened him and his army with death during his discourse with Leo, and this prompted Attila to submit to his request.
Leo's intercession could not prevent the sack of Rome by the Vandal King Genseric three years later, but murder and arson were repressed by his influence. The Pope and members of his clergy, went to meet the invader to implore him to desist. While the Vandals plundered the city, the gesture nevertheless prevented Rome from being burned and assured that the Basilicas of St Peter, St Paul and St John, in which part of the terrified population sought refuge, were spared. Leo was also a strong theologian and accomplished writer, almost 100 sermons and 150 letters of Leo I have been preserved. The most significant piece of writing is known as ‘Leo’s Tome’ which was written for the Second Council of Ephesus. Leo's representatives delivered his famous Tome, which was a statement of the faith of the Roman Church which repeats, in close adherence to Augustine of Hippo, the formulas of western Christology. The council did not read the letter nor did it pay any attention to the protests of Leo's which is why the council was never recognized as ecumenical and was later repudiated by the Council of Chalcedon. However Leo’s Tome is seen as deeply influential on the Church’s Christology, with Leo defends the true divinity and the true humanity of the one Christ against heretical one-sidedness
Another significant theological legacy of Leo was his suppression of Manicheism, a form of religious Dualism. Which was spreadinhg with extraordinary rapidity in the Church in both East and West. It was a religion founded by the Persian Mani in the latter half of the third century and purported to be the true synthesis of all the religious systems then known, with Zoroastrian Dualism, Babylonian folklore, Buddhist ethics, and some small and superficial, additions of Christian elements. Mani was the final prophet after Zoroaster, Gautama Buddha, and Jesus. It taught an elaborate dualistic cosmology describing the struggle between a good, spiritual world of light, and an evil, material world of darkness. Through an ongoing process that takes place in human history, light is gradually removed from the world of matter and returned to the world of light, whence it came. Constantly downplaying the incarnation – it would create an elite secret group withing communities who had access to hidden knowledge – gnosis. Manichaeans fleeing the Vandals had come to Rome in 439 and secretly organized there; Leo learned of it around 443, and proceeded against them by holding a public debate with their representatives and writing letters of warning to the Italian bishops. It was a seductive and persistent heresy
Pope Benedict XVI, an important theologian in his own right said that Leo's papacy "...was undoubtedly one of the most important in the Church's history."