Saint Leo The Great (b. 400, died 461 ad) the most significant Pope to guide the Universal Church during a time when the Roman Empire was collapsing. Pope Leo I was a wise individual that brilliantly and energetically confronted extraordinary challenges from barbarians, heresy, Roman authority and Dogmatic doctrine. Pope Leo's twenty-one year papacy (440-461 ad) in the middle of the fifth century earned him the title 'Leo The Great'. One of only two Popes with that distinction. His legacy comprises 96 sermons and 143 letters. One of only thirty-three Saints to be called Doctor of the Church.
Early Christian life
Little is known about the man's early life. St. Leo hails from the Tuscany. Well educated, he becomes a Deacon in the Roman Church during the pontificate of Pope Celestine I. As Deacon, Leo was sent by the Imperial Court under authority of Roman Emperor Valentinian III, with significant task of treatise. He was to reconcile a dispute between the chief magistrate and the provinces military commander. This would be the first of many successful negotiations he commanded throughout his Christian ministry. During this time, Pope Sixtus III died and St. Leo I was unanimously chosen as Bishop of Rome in September 29, 440.
According to Pope Leo, the Church is built upon Peter and the promise from scripture, Matthew 16:16-19. Everything is of Christ and Peter's role embodies the Roman Church. What is held in common with Christ, Peter and the apostles held through Pope Leo and is true of his successors. Pope Leo urges strict observance of the canons and the disciplines of the Roman Church. Thus, the Church manifested itself by Pope Leo in the most distinctive of ways. He obtained an Imperial script that confirmed authority of the Pope over his bishops, in unity with the Emporer. Ruled that the bishops are charged with the care of their own special regions, with Rome leadership. The decline of the Empire, the breakdown of morality and disorder from barbarians disrupted all conditions of life. Leo's sermons, letters and edicts would define Christ and be used against heretics and others threatening Rome. Unity of Western Christology was his goal during a time of great upheaval.
Throughout his entire papacy, Pope Leo felt compelled to combat heretics. Gauls, Pelagians corrupted life. His own church had deviated in Constantinople and Sicily, which earned strong rebukes from Leo. The doctrine of the Incarnation was devised out of his dealing with Constantinople. Pope Leo I convened the Fourth Ecumenical Council, The Council of Chalcedon in 451. This was a major factor in condemning heresy. An expression of Catholic Faith concerning the Person of Christ.  First to define Christ as one person, two natures, divine and human. The Tome to Flavian became a significant achievement. Leo the Great of Rome, The Tome to Flavian
Manicheans were persecuted by the Vandals. They fled from North Africa to Rome in 439 and secretly organized their religion during Pope Leo's reign. Leo learned of this around 443 and warned the early Christians to report on them for their heresy. On June 19, 445 Roman Emporer Valentinian III was instructed by Pope Leo to convert, punish or drive out Mancheans.
In Spain, the heresy of Priscillianism had been attracting dissent. Leo insisted on the exact observance of the ecclesiastical precepts and refuted their heresy. Pope Leo convened all neighbouring provincial bishops to determine whether any had become tainted with the poison of this heresy. Should any such be discovered, they were to be excommunicated without hesitation.
Attila the Hun, who was called the Scourge of God, had invaded Italy. Filled with anger, for three years his troops marched on and destroyed cities on their way to Rome. In 452, Pope Leo sought out to reconcile with Attila before they entered Rome. In face to face discussions, an image of Peter with a sword appeared overhead of Leo, spooking Atilla. The Pope persuaded him to turn back.
Roman dominion on Latin North Africa forced Vandals to enter Rome. Three years after the Hun, the Vandal Genseric fought the empire. Again Leo, by Godly power and Holy authority, persuaded Genseric to stop the destruction and slaughter.