At Rome, Pope St. Celestine I

who condemned Nestorius, Bishop of Constantinople, and put Pelagius to flight. By his command the holy Universal Council of Ephesus was held to condemn the same Nestorius. (from the Martyrology)

Definition of the faith at Nicaea [6th session 22 July 431]

The synod of Nicaea produced this creed: I believe ....
Credo in unum Deum, Patrem omnipotentem, factorem caeli et terrae, visibilium omnium et invisibilium.

Et in unum Dominum Iesum Christum, Filium Dei unigenitum, et ex Patre natum ante omnia saecula. Deum de Deo, Lumen de Lumine, Deum verum de Deo vero, genitum non factum, consubstantialem Patri; per quem omnia facta sunt. Qui propter nos homines et propter nostram salutem descendit de caelis. Et incarnatus est de Spiritu Sancto ex Maria Virgine, et homo factus est. Crucifixus etiam pro nobis sub Pontio Pilato, passus et sepultus est, et resurrexit tertia die, secundum Scripturas, et ascendit in caelum, sedet ad dexteram Patris. Et iterum venturus est cum gloria, iudicare vivos et mortuos, cuius regni non erit finis.

Et in Spiritum Sanctum, Dominum et vivificantem, qui ex Patre (Filioque) procedit. Qui cum Patre et Filio simul adoratur et conglorificatur: qui locutus est per prophetas. Et unam, sanctam, catholicam et apostolicam Ecclesiam. Confiteor unum baptisma in remissionem peccatorum. Et expecto resurrectionem mortuorum, et vitam venturi saeculi. Amen.

It seems fitting that all should assent to this holy creed. It is pious and sufficiently helpful for the whole world. But since some pretend to confess and accept it, while at the same time distorting the force of its expressions to their own opinion and so evading the truth, being sons of error and children of destruction, it has proved necessary to add testimonies from the holy and orthodox fathers that can fill out the meaning they have given to the words and their courage in proclaiming it. All those who have a clear and blameless faith will understand, interpret and proclaim it in this way.

When these documents had been read out, the holy synod decreed the following.

1. It is not permitted to produce or write or compose any other creed except the one which was defined by the holy fathers who were gathered together in the holy Spirit at Nicaea.

2. Any who dare to compose or bring forth or produce another creed for the benefit of those who wish to turn from Hellenism or Judaism or some other heresy to the knowledge of the truth, if they are bishops or clerics they should be deprived of their respective charges and if they are laymen they are to be anathematised.

3. In the same way if any should be discovered, whether bishops, clergy or laity, thinking or teaching the views expressed in his statement by the priest Charisius about the Incarnation of the only-begotten Son of God or the disgusting, perverted views of Nestorius, which underlie them, these should be subject to the condemnation of this holy and ecumenical synod. A bishop clearly is to be stripped of his bishopric and deposed, a cleric to be deposed from the clergy, and a lay person is to be anathematised, as was said before.