Under Leo, he became the principal papal secretary and on a trip through Apulia in 1053, he received from John, Bishop of Trani the letter from Leo, Archbishop of Ochrid, criticising Western rites and practice. He translated the Greek letter into Latin and gave it to the pope, who ordered a response drawn up. This exchange led to Humbert being sent at the head of a legatine mission with Frederick of Lorraine, later Pope Stephen IX, and Peter, archbishop of Amalfi, to Constantinople to confront PatriarchMichael Cerularius. He was cordially welcomed by the EmperorConstantine IX, but spurned by the patriarch. Eventually, on 16 July1054, despite the fact that Leo had died and the excommunication was invalid, he laid the excommunication on the high altar of the church of the Hagia Sophia during the celebration of the liturgy. This caused the Great Schism and marked the official separation of the Roman Church from the Orthodox Church.
In his later years, he was made librarian of the Roman Curia by Stephen IX, his former legatine companion, and he penned the reform treatise Lib tres adversus Simoniacos ('Three Books Against the Simoniacs') (1057), which helped initiate the Gregorian Reform movement. He is also credited as the brains behind the electoral decree of 1059, which stated that popes would henceforth be elected by the College of Cardinals.
Sources
Norwich, John Julius (1967). The Normans in the South 1016–1130. London: Longman.