Leo reigned as the pope of Rome in the middle of the 5th century. For better and for worse, his impact on the faith and shape of the church is still felt today. During his lifetime the church at large was wrestling with a very important question: How are we to understand the human nature and divine nature of Christ and what does this mean about His person? Many parts of the church were thinking about this question and some teachings that were emerging were either contradictory to the record of the Bible or emphasized his humanity or divinity to the exclusion of the other. Since Jesus is at the heart of Christianity, thinking rightly about Him is of great importance to every believer. Because of this, hundreds of church leaders gathered together for a council near modern day Istanbul to discuss the matter and to seek a unity in thinking and a uniformity in teaching this doctrine.
To this event, Leo sent a short essay known today as the Tome which spelled out that Christ Jesus was fully God and fully man in one person and that he had a divine/human nature in which he bore the characteristics of God the Father and that of Mary with the exception of not having a sin-nature. The Tome was so well received that it pretty much ended all discussion on the matter and there was a sense by all who participated that God had spoken to the council through Leo. This resulted in what is known as the Chalcedonian Definition of Christ which has been the teaching of the Church ever since (you can read this in full at http://en.wikipedia.org/wiki/Chalcedonian_Definition).
Leo was also the person who fully developed the idea that the pope of Rome (pope meaning ‘father of the faith’) was the actual successor of St. Peter. This belief, known as the Petrine Idea, takes the words of Jesus to Peter in Matthew 16:18 to mean that the future church will be built upon Peter the apostle. Protestants have always taken this verse to mean the church will be built upon Peter’s confession that Jesus is the Christ for good reason, but for Leo and many others the Petrine Idea was an indisputable fact. One part of this could literally be true as St. Peter’s Basilica is built over a first century graveyard and at the center is the place believed to be where Peter was buried after he was martyred by the Romans. But Leo took this to mean that the Apostleship of Peter would be passed down to each Pope of the church of Rome and that when they wrote, taught, or spoke it was actually Peter who was speaking through them. This also meant that since Peter was the appointed foundation, that his successors would be kept from errors in doctrine.
While Leo was a bit overly enthusiastic about the apostle Peter and his own relationship to him as bishop of Rome, Leo was completely right in his thinking about Christ and has helped the church through most of its history to have a sound theology of Jesus of Nazareth. With both ideas alive and well after 1600 years, Pope Leo I was truly one of the shapers of the Christian faith.