Consecrated on Sept. 29, 440, as successor to St. Sixtus III, Leo, one of the few popes termed great, immediately worked to suppress heresy, which he regarded as the cause of corruption and disunity. Yet his most significant theological achievement was not his negative suppression of heresy but his positive formulation of orthodoxy.

His treatment of the monk Eutyches of Constantinople provides an example. The monk had founded Eutychianism (Eutychian), an extreme form of monophysitism holding that Christ had only one nature, his human nature being absorbed in his divine nature. Patriarch Flavian of Constantinople excommunicated Eutyches, who then appealed to Leo. After examining the case, Leo sent Flavian (449) his celebrated Tome, which rejected Eutyches' teaching and presented a precise, systematic doctrine of Christ's Incarnation and of the union of both his natures. The Council (451) of Chalcedon (modern Kadikoy, Turkey), summoned to condemn Eutychianism, declared that Leo's Tome was the ultimate truth. Furthermore, the council recognized Leo's doctrine as “the voice of Peter.” Thus for the church Leo's Tome established the doctrine that Christ's natures coexist and his Incarnation reveals how human nature is restored to perfect unity with divine, or absolute, being.

Leo's 432 letters and 96 sermons expound his precept of papal primacy in church jurisdiction. He held that papal power was granted by Christ to St. Peter alone, and that that power was passed on by Peter to his successors. In one letter, for example, he cautioned the Bishop of Thessalonica that although he had been entrusted with office and shared Leo's solicitude, he was “not to possess the plenitude of power.”

Leo further enhanced the prestige of the papacy and helped to place Western leadership in its hands by dealing with invading barbaric tribes. He persuaded the Huns, a nomadic people terrorizing northern Italy, not to attack Rome (452), and the Vandals, a Germanic people, not to sack Rome when they occupied it three years later. Leo was declared a doctor of the church by Pope Benedict XIV in 1754.