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Why did Pope Leo I write the Tome of Leo?

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Christology The Transfiguration, the nature of Jesus as the Son of God being revealed to the apostles Peter, James, and John, mosaic icon, early 13th century; in the Louvre, Paris.

Pope Leo I wrote the Tome (or Dogmatic Letter) in 449 to address the rise of Eutychianism, an extreme form of monophysitism promoted by the monk Eutyches of Constantinople.12

Eutyches taught that Christ possessed only one nature, arguing that his human nature was absorbed into his divine nature after the Incarnation. After Eutyches was excommunicated by Patriarch Flavian of Constantinople, he appealed his case to the Pope.12

In response, Leo examined the matter and issued the Tome to Flavian, which served as a formal rejection of Eutyches’ teachings. The document provided a precise, systematic defense of orthodoxy, affirming that Christ exists in two complete, distinct natures—human and divine—united without confusion in one person. The Council of Chalcedon later endorsed the Tome in 451, recognizing it as a definitive statement of faith and the voice of St. Peter.1234