Leo was a Roman deacon and a diplomat of the Western imperial court before his election as Bishop of Rome in 440. From the start of his twenty-one-year pontificate he saw the papacy as the universal pastoral office Christ had given to Peter — and articulated that conviction with a clarity and authority no predecessor had matched.
His Tome to Flavian (449) — sent to the Council of Chalcedon in 451 — gave the universal Church the precise formula by which the divinity and humanity of Christ are united: 'one and the same Christ, Son, Lord, only-begotten, acknowledged in two natures, without confusion, change, division, or separation.' When the Tome was read at Chalcedon, the assembled bishops are reported to have acclaimed: 'Peter has spoken through Leo!' The Council adopted the Tome substantially as its Christological definition.
In 452 Leo went out from Rome to meet Attila the Hun at the Mincio River near Mantua, persuading the conqueror to turn back from a planned sack of Rome — a meeting commemorated in Raphael's great fresco in the Stanze of the Vatican. Three years later, when the Vandals under Gaiseric did reach Rome (455), Leo went out again — this time obtaining at least the agreement that the city would not be burned and the population not massacred.
He left behind 96 sermons and 143 letters of the highest theological and literary quality. He was named Doctor of the Church by Benedict XIV in 1754 — only the second pope (after Gregory the Great) so honoured.