What will hinder me from referring to other Fathers? Leo the Great, whilst bishop of [Old] Rome, carefully demonstrated divine matters in his inspired and dogmatic Tome. In this, he was confirmed by the Fourth Synod. He confirmed its decree, and was praised by the sacred, and God-inspired assembly. He clearly taught that the All-Holy Spirit proceeds from the Father. He thus radiates the very same light of Orthodoxy, not only upon the entire West, but also to the ends of the East through his God-inspired and dogmatic epistles, through the legates who exercised his authority, and through the peace with which he illumined that great assembly collected by God. Moreover, he also said that if anyone set up or teach another doctrine other than that taught by the Synod, that person should be deposed if he were of the dignity of the priesthood or anathematised if he were a layperson or even a monastic, religious or ascetic. Whatever that God-inspired Synod decreed, Leo, similarly inspired by God, openly confirmed through the holy men Paschasinus, Lucentius and Boniface (as one may hear many times from them, indeed not only from them, but from him who sent them). Dispatching synodical letters, Leo himself testifies and confirms that the speeches, spirit, and decisions of his delegates are not theirs, but his own. Still, even if there were nothing of this, it is sufficient that they were his representatives at the Synod and that when the Synod ended, he professed to abide by its decisions.
…Thus, I will not hold back what needs to be said: it is the same senseless act of impiety which rushes towards perdition instead of towards the Savior. With a multi-tongued voice under the inspiration of the Spirit, the Synod spoke clearly; they are confirmed by all votes and the all-wise Leo resoundingly concurs. Apply your mind, therefore, to what follows towards the end of the entire section of the Acts it says quite clearly: The Holy and Ecumenical Synod fixes therefore with these men from every quarter, with exactness and harmony, our exact exposition, the meaning of which the chief legate of Leo procured. What did it decree? That no one is permitted to declare a different faith; that is to say, neither to write it, nor assent to it, nor think it, nor teach it to others. But for those who presume to accept another faith, that is they who promulgate or teach or deliver a different Symbol to those who wish to return to the knowledge of the truth from Hellenism, or Judaism, or any other heresy; and if any are bishops or clergy, let the bishops be deprived of their diocese and the clerics be deposed of their office; but if they be monks or laity, let them be anathema.
You should consider the equally renowned Vigilius, equal in throne and rank of glory with those other men, who assisted at the Fifth Synod which is also adorned with holy and ecumenical decrees. Like an unerring rule, this man conformed himself to its true dogmas. He voiced agreement in other matters and with equal zeal matching those Fathers before him and of his own time, proclaiming that the All-Holy and Consubstantial Spirit proceeds from the Father, also saying that if anyone introduced any definition other than the unanimous and common faith of the pious, then he should be delivered to the same bonds of anathema.
You should consider the noble and good Agatho, honoured with the same victorious deeds. Through his legates, he convened and made illustrious the Sixth Synod (which also shines with ecumenical rank), being present there, if not bodily, then certainly in will and with all diligence. He preserved the Symbol of our inviolate, pure, and unchangeable Faith without innovation, in accordance with the synods. Moreover, he confirmed the Synod by placing under an equal curse any so bold as to alter any word taught by it as dogma; these words which were affirmed as dogma from the beginning. (Mystagogy of the Holy Spirit, 79-83)