St. Cyril is one of the great Greek fathers of the Church. He was chosen by divine Providence to be the shield and champion of the Church against Nestorius, who denied the unity of person in Christ. If this heresy had succeeded, Mary would not be called the Mother of God.
Excepting Sts. Athanasius and Augustine, his equal as a defender of orthodoxy can hardly be found in the Church's history. His greatest achievement was the successful direction of the ecumenical council at Ephesus (431), of which he was the soul (Pope Celestine had appointed him papal legate). In this council two important dogmas were defined, viz., that there is but one person in Christ, and that Mary in the literal sense of the word can be called the Mother of God (Theotokos). His successful defense of the latter doctrine is his greatest title to honor.
His writings show such depth and clarity that the Greeks called him the "seal of the fathers." He died in 444 A.D., after having been bishop for thirty-two years. In Rome, the basilica of St. Mary Major stands as a most venerable monument to the honor paid Mary at the Council of Ephesus. On the arch leading into the sanctuary important incidents in the lives of Jesus and Mary are depicted in mosaic.
~Excerpted from The Church's Year of Grace, Pius Parsch