15 Moreover, he plainly saw - as may be gathered easily from a reading of his letters on the subject of the Antiochene Bishops - that this Christian faith, which we must at all costs preserve and protect, has been delivered to us through the Sacred Scriptures and through the teaching of the Holy Fathers, and is clearly and authentically set forth by the living and infallible teaching authority of the Church. Thus, when the Bishops of the Province of Antioch claimed that for the restoration and maintenance of peace it was enough if they kept the faith of the Council of Nicaea, St. Cyril, while himself firmly adhering to the Nicene Creed, also required of his brethren in the Episcopate, as a condition of reunion, that they should reject and condemn the Nestorian heresy. For he quite well understood that it is not enough to accept willingly the ancient pronouncements of the teaching office of the Church, but that it is also necessary to believe humbly and loyally all that is subsequently enjoined upon our faith by the Church in virtue of her supreme authority.
Source: Orientalis Ecclesiae (Vatican.va)