He was tonsured as a monk in 1420 and is considered to have been the confessor of Emperor
John VIII Palaiologos. He was a supporter of the Union with the
Catholic Church. He played a very active role in the theological discussions. He participated in the preliminary negotiations with Rome at the
Council of Florence and later accompanied Patriarch
Joseph II of Constantinople to the Council of Florence, where he also represented
Philotheos of Alexandria. He was elected Patriarch after the death of the also-unionist Patriarch
Metrophanes II of Constantinople. Gregory III did his best to reconcile monks, the church hierarchy, and common people to the agreement reached at Florence, but in vain. He was opposed by
Gennadius Scholarius and
John Eugenikos, who wrote extensively against the council. Leading anti-Unionist clergy refused to pray for the Emperor in their churches. In 1450, the tension in ecclesiastical circles grew so tense that Gregory III left his post and arrived in
Rome in August 1451 (less than two years before the
fall of Constantinople). He was cordially received by
Pope Nicholas V, who aided him financially. Pro-unionists in the
Latin-occupied areas of Greece continued to consider him the legitimate patriarch of Constantinople. == Legacy ==