Leo was the Archdeacon Roman bishop Coelestin, an excellent preacher, a well known and respected cleric and diplomat. 440 He was later Coelestins successor. According to legend, it is Peter himself over the Pallium have: Leo worked strongly to the view of the Bishop of Rome, manage the "Chair of Peter, his task was to" care about the sheep entrusted to him "

It was the time of migration and the break under their onslaught Roman Empire and also the time innerkirchlich hard struggle for the right faith: Arianism, Nestorianismus, Manichaeism, and Gnosis Pelagianismus made to the church. In fierce clashes with the Eastern Church took Leo the doctrine of two natures of Christ. When the 449 East Church on a synod in Ephesus on Monophysitismus stated, damned Leo this Assembly as a "robber synod". Leos' Tomus "called doctrinal letter to the Patriarch Flavian of Constantinople from 449 this year was 451 at the Council of Chalkedon with the remark, Peter himself had spoken through Leo, the council approved. The council was convened to the false doctrine of Eutychianismus, a form of Monophysitismus to condemn; Leos legates had the presidency and could be enforced with the help of the Byzantine emperor, the Patriarch of Alexandria and the fallen Roman doctrine to prevail.

 
Alessandro Algardi: grave monument of Leo I. Leo (left) points to the heavenly assistance of Peter, Attila (right) differs back, marble, 1646-53, in St Peter in the Vatican
In many countries, Leo ordered the new church organization and strengthened the church hierarchy. He decisively strengthened the position of the bishop of Rome and can be considered the first real "pope" with power to manage the entire Western church to be. As Bishop of Rome, he was anxious, a national responsibility to build and "Princeps Apostolorum", "First of the Apostles", power of attorney over the other bishops and the entire church to exercise. Cemented his power by the 451 he convened a Synod of Bishops in Milan, the city with the most powerful next to the bishop of Rome. When he was at a synod in Rome by Bishop Hilary of Arles == attack, which proved to be independent of Rome Metropolitan of Gaul wanted to confirm, let Leo by imperial decree the right of residence in his diocese limit.

So Leo's supremacy over all bishops of the Church in the West by as vicarius Christ "was the office-holders in Rome, the concern for the whole church transferred. The (fake) 6 Canon of Nicäa, overwritten "De ecclesiae primatu Romana", "on the primacy of the Roman Church," begins with the words "Ecclesia Romana semper habuit primatum", "the Church of Rome had always been the priority."

452 Leo traveled einmarschierenden the Huns under Attila contrary, in Mantua, he was against Attila so bravely and with dignity occurred such that this addition to Leo Paul and Peter with sword compatriots have gezücktem; Leo reached the reversal of the Hun and saved Rome from destruction and looting . Three years later, Rome was plundered by the Vandals under Geiserich, the emperor was already dead, fled the Roman army, Rome an open city, Leo joined the tradition even now after the Vandals opposite occurs, and could not to prevent the looting of the city, Geiserich but by the commitment given that citizens are not murdered. The Vandals were restricted to prey abundance and art - so they covered the gilded roof of the Temple of Jupiter - and abducted thousands of hostages - including the emperor's widow Eudokia - to ransom demand to their actions coined the term "vandalism", but that life of the Romans remained: why was Leo as the savior of Rome and has been highly revered during his lifetime.