IV. Those who are still in error must be thoroughly instructed in the historic Faith.
But how many are the authorities, both in the New and Old Testaments, by which this truth is declared, as befits the antiquity of your See, you clearly understand, seeing that the belief of the Fathers and my letter written to Flavian, of holy memory, of which you yourself made mention, confirmed, as they have been, by the universal synod, are sufficient for you. And therefore it behooves you, beloved, to take heed that no one raise a murmur against the unspeakable mystery of our Redemption and Hope. But if there are any who are still in the darkness of ignorance or the discord of perversity, let them be instructed by the authority of those whose doctrine in God's Church was and clear, that they may recognize that on the Incarnation of God's Word we believe what they did, and may not by their obstinacy place themselves outside the Body of Christ, in which we died and rose with Him: because neither loyalty to the Faith nor the plan of the mystery admits that either the Godhead should be possible in its own essence, or the reality be falsified in His taking on Him of our flesh. Dated 4th September, in the consulship of the illustrious Aetius and Studius (454).
Source: Letters (New Advent)