Besides, since with the heart man believes unto righteousness, and with the mouth confession is made unto salvation, I confess that I receive and revere, as the four books of the Gospel so also the four Councils: to wit, the Nicene, in which the perverse doctrine of Arius is overthrown; the Constantinopolitan also, in which the error of Eunomius and Macedonius is refuted; further, the first Ephesine, in which the impiety of Nestorius is condemned; and the Chalcedonian, in which the pravity of Eutyches and Dioscorus is reprobated.
These with full devotion I embrace, and adhere to with most entire approval; since on them, as on a four-square stone, rises the structure of the holy faith; and whosoever, of whatever life and behaviour he may be, holds not fast to their solidity, even though he is seen to be a stone, yet he lies outside the building. The fifth council also I equally venerate, in which the epistle which is called that of Ibas, full of error, is reprobated; Theodorus, who divides the Mediator between God and men into two subsistences, is convicted of having fallen into the perfidy of impiety; and the writings of Theodoritus, in which the faith of the blessed Cyril is impugned, are refuted as having been published with the daring of madness.
But all persons whom the aforesaid venerable Councils repudiate I repudiate; those whom they venerate I embrace; since, they having been constituted by universal consent, he overthrows not them but himself, whosoever presumes either to loose those whom they bind, or to bind those whom they loose. Whosoever, therefore, thinks otherwise, let him be anathema. But whosoever holds the faith of the aforesaid synods, peace be to him from God the Father, through Jesus Christ His Son, Who lives and reigns consubstantially God with Him in the Unity of the Holy Spirit for ever and ever. Amen.
Source: Register of Letters (New Advent)