I. The decrees of Chalcedon and Nicæa are identical and final.
With much joy my mind exults in the Lord, and great is my cause for thankfulness, now that I perceive your clemency's most excellent faith to be in all things enlarged by the gifts of heavenly grace, and I experience by increased diligence the devotion of a priestly mind in you. For in your Majesty's communications it is beyond doubt revealed what the Holy Spirit is working through you for the good of the whole Church, and how greatly it is to be desired by the prayers of all the faithful that your empire may be everywhere extended with glory, seeing that besides your care for things temporal you so perseveringly exercise a religious foresight in the service of what is divine and eternal: to wit that the Catholic Faith, which alone gives life to and alone hallows mankind, may abide in the one confession, and the dissensions which spring from the variety of earthly opinions may be driven away, most glorious Emperor, from that solid Rock, on which the city of God is built.
And these gifts of God will at last be granted us from Him, if we be not found ungrateful for what has been vouchsafed, and as though what we have gained were naught, we seek not rather the very opposite. For to seek what has been discovered, to reconsider what has been completed, and to demolish what has been defined, what else is it but to return no thanks for things gained and to indulge the unholy longings of deadly lust on the food of the forbidden tree? And hence by deigning to show a more careful regard for the peace of the universal Church, you manifestly recognize what is the design of the heretics' mighty intrigues that a more careful discussion should take place between the disciples of Eutyches and Dioscorus and the emissary of the Apostolic See, as if nothing had already been defined, and that what with the glad approval of the Catholic priests of the whole world was determined at the holy Synod of Chalcedon should be rendered invalid to the detriment also of the most sacred Council of Nicæa.
For what in our own days at Chalcedon was determined concerning our Lord Jesus Christ's Incarnation, was also so defined at Nicæa by that mystic number of Fathers, lest the confession of Catholics should believe that God's Only-begotten Son was in anything unequal to the Father, or that when He was made Son of man He had not the true nature of our flesh and soul.
Source: Letters (New Advent)