Dogmatic Definition of the Council of Chalcedon, 451 A.D. (entire)
The sacred and great and universal synod by God's grace and by decree of your most religious and Christ-loving emperors Valentinian Augustus and Marcian Augustus assembled in Chalcedon, metropolis of the province of Bithynia, in the shrine of the saintly and triumphant martyr Euphemia, issues the following decrees.
In establishing his disciples in the knowledge of the faith, our lord and saviour Christ said: "My peace I give you, my peace I leave to you"', so that no one should disagree with his neighbour regarding religious doctrines but that the proclamation of the truth would be uniformly presented. But the evil one never stops trying to smother the seeds of religion with his own tares and is for ever inventing some novelty or other against the truth; so the Master, exercising his usual care for the human race, roused this religious and most faithful emperor to zealous action, and summoned to himself the leaders of the priesthood from everywhere, so that through the working of the grace of Christ, the master of all of us, every injurious falsehood might be staved off from the sheep of Christ and they might be fattened on fresh growths of the truth.
This is in fact what we have done. We have driven off erroneous doctrines by our collective resolution and we have renewed the unerring creed of the fathers. We have proclaimed to all the creed of the 318; and we have made our own those fathers who accepted this agreed statement of religion -- the 150 who later met in great Constantinople and themselves set their seal to the same creed.
Therefore, whilst we also stand by the decisions and all the formulas relating to the creed from the sacred synod which took place formerly at Ephesus, whose leaders of most holy memory were Celestine of Rome and Cyril of Alexandria we decree that pre-eminence belongs to the exposition of the right and spotless creed of the 318 saintly and blessed fathers who were assembled at Nicaea when Constantine of pious memory was emperor: and that those decrees also remain in force which were issued in Constantinople by the 150 holy fathers in order to destroy the heresies then rife and to confirm this same catholic and apostolic creed: the creed of the 318 fathers at Nicaea, and the same of the 150 saintly fathers assembled in Constantinople.
This wise and saving creed, the gift of divine grace, was sufficient for a perfect understanding and establishment of religion. For its teaching about the Father and the Son and the holy Spirit is complete, and it sets out the Lord's becoming human to those who faithfully accept it.
But there are those who are trying to ruin the proclamation of the truth, and through their private heresies they have spawned novel formulas: some by daring to corrupt the mystery of the Lord's economy on our behalf, and refusing to apply the word "God-bearer" to the Virgin; and others by introducing a confusion and mixture, and mindlessly imagining that there is a single nature of the flesh and the divinity, and fantastically supposing that in the confusion the divine nature of the Only-begotten is passible.
Therefore this sacred and great and universal synod, now in session, in its desire to exclude all their tricks against the truth, and teaching what has been unshakeable in the proclamation from the beginning, decrees that the creed of the 318 fathers is, above all else, to remain inviolate. And because of those who oppose the holy Spirit, it ratifies the teaching about the being of the holy Spirit handed down by the 150 saintly fathers who met some time later in the imperial city--the teaching they made known to all, not introducing anything left out by their predecessors, but clarifying their ideas about the holy Spirit by the use of scriptural testimonies against those who were trying to do away with his sovereignty.
And because of those who are attempting to corrupt the mystery of the economy and are shamelessly and foolishly asserting that he who was born of the holy virgin Mary was a mere man, it has accepted the synodical letters of the blessed Cyril, [already accepted by the Council of Ephesus] pastor of the church in Alexandria, to Nestorius and to the Orientals, as being well-suited to refuting Nestorius's mad folly and to providing an interpretation for those who in their religious zeal might desire understanding of the saving creed.
To these it has suitably added, against false believers and for the establishment of orthodox doctrines the letter of the primate of greatest and older Rome, the most blessed and most saintly Archbishop Leo, written to the sainted Archbishop Flavian to put down Eutyches's evil-mindedness, because it is in agreement with great Peter's confession and represents a support we have in common.
It is opposed to those who attempt to tear apart the mystery of the economy into a duality of sons; and it expels from the assembly of the priests those who dare to say that the divinity of the Only-begotten is passible, and it stands opposed to those who imagine a mixture or confusion between the two natures of Christ; and it expels those who have the mad idea that the servant-form he took from us is of a heavenly or some other kind of being; and it anathematises those who concoct two natures of the Lord before the union but imagine a single one after the union.
So, following the saintly fathers, we all with one voice teach the confession of one and the same Son, our Lord Jesus Christ: the same perfect in divinity and perfect in humanity, the same truly God and truly man, of a rational soul and a body; consubstantial with the Father as regards his divinity, and the same consubstantial with us as regards his humanity; like us in all respects except for sin; begotten before the ages from the Father as regards his divinity, and in the last days the same for us and for our salvation from Mary, the virgin God-bearer as regards his humanity; one and the same Christ, Son, Lord, only-begotten, acknowledged in two natures which undergo no confusion, no change, no division, no separation; at no point was the difference between the natures taken away through the union, but rather the property of both natures is preserved and comes together into a single person and a single subsistent being; he is not parted or divided into two persons, but is one and the same only-begotten Son, God, Word, Lord Jesus Christ, just as the prophets taught from the beginning about him, and as the Lord Jesus Christ himself instructed us, and as the creed of the fathers handed it down to us.
Since we have formulated these things with all possible accuracy and attention, the sacred and universal synod decreed that no one is permitted to produce, or even to write down or compose, any other creed or to think or teach otherwise. As for those who dare either to compose another creed or even to promulgate or teach or hand down another creed for those who wish to convert to a recognition of the truth from Hellenism or from Judaism, or from any kind of heresy at all: if they be bishops or clerics, the bishops are to be deposed from the episcopacy and the clerics from the clergy; if they be monks or layfolk, they are to be anathematised.