The significance of Leo’s pontificate lies in his assertion of the universal jurisdiction of the Roman bishop, as expressed in his letters, and still more in his 96 extant orations. This assertion is commonly referred to as the doctrine of Petrine supremacy.

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According to Leo and several Church Fathers, as well as certain interpretations of the Scriptures, the Church is built upon Peter, in pursuance of the promise of Matthew 16:16–19. Peter participates in everything which is Christ’s; what the other apostles have in common with him they have through him. What is true of Peter is true also of his successors. Every other bishop is charged with the care of his particular flock, the Roman pontiff with that of the whole Church. Other bishops are his assistants in this great task. In Leo’s eyes the decrees of the Council of Chalcedon acquired their validity from his confirmation.

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Leo’s letters and sermons reflect the many aspects of his career and personality,and are invaluable historical sources. His rhythmic prose style, called cursus leonicus, influenced ecclesiastical language for centuries.

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The Roman Catholic Church marks 10 November as the feast day of Saint Leo, given in the Martyrologium Hieronymianum and the 8th-century Calendar of Saint Willibrord as the date of his death and entry to heaven. His feast was once celebrated in Rome on 28 June, the anniversary of the placing of his relics in Saint Peter’s Basilica, but in the 12th century, the Gallican Rite feast of 11 April was admitted to the General Roman Calendar, which maintained that date until 1969. Some traditionalist Catholics continue to observe pre-1970 versions of that calendar.

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The Eastern Catholic Churches as well as the Eastern Orthodox Church celebrate Saint Leo on 18 February.