These were difficult times for the Church, when [[Heresy|heretic]]s assaulted Orthodoxy with their false teachings. Leo combined pastoral attentiveness with uncompromising firmness in the confession of the Faith. He was in particular one of the basic defenders of Orthodoxy against the heresies of [[Eutyches]] and [[Dioscorus of Alexandria]], who taught that there was only one nature in the Lord [[Jesus Christ]] this is denied by the[[Oriental Orthodxy}} churches as they consider [[Eutyches]]'s idea a herecy as the other churches but to have redemed himself by retreving this herecy in the Ephesus second council <ref>Story of the Coptic church by [[Iris Habib elmasry]] Volume I</ref>. He was also a defender against the heresy of [[Nestorius]].

At the Council of Chalcedon, at which 630 bishops were present, a letter of Leo to the deceased St. [[Flavian of Constantinople|Flavian]], [[Patriarch of Constantinople]] (447-449), was read. Flavian had suffered for Orthodoxy under the [[Robber Council of Ephesus]] in the year 449. In the letter of Leo, the Orthodox teaching about the two natures of Christ, divine and human, was set forth. All the bishops present at the Council were in agreement with this teaching, and so the heretics Eutyches and Dioscorus were [[excommunicate]]d from the Church.