FTP name this doctor of the church and pope from 440 to 461.
After finding out that he was chosen as the new Pope while on a mission to Gaul, he worked to suppress heresy, especially extreme forms of monophysitism.  The Council of Chalcedon, which he summoned, accepted his teachings that Christ is both fully divine and fully human, both incarnated in an historical person, as ultimate truth.  The following year, in 452, he personally convinced Attila and the Huns not to sack the city of Rome.  FTP name this doctor of the church and pope from 440 to 461.
After finding out that he was chosen as the new Pope while on a mission to Gaul, he worked to suppress heresy, especially extreme forms of monophysitism.  
Pope Leo I or the Great (prompt on "Leo")