The Fifth Ecumenical Council, The Sentence of the Synod
"...And it was shewn in the acts that in former times Ibas had been accused because of the very impiety which is contained in this letter; at first by Proclus, of holy memory, the bishop of Constantinople, and afterwards by Theodosius, of pious memory, and by Flavian, who was ordained bishop in succession to Proclus, who delegated the examination of the matter to Photius, bishop of Tyre, and to Eustathius, bishop of the city of Beyroot. Afterwards the same Ibas, being found guilty, was cast out of his bishopric. Such was the state of the case, how could anyone presume to say that that impious letter was received by the holy council of Chalcedon and that the holy council of Chalcedon agreed with it throughout? Nevertheless in order that they who thus calumniate the holy council of Chalcedon may have no further opportunity of doing so, we ordered to be recited the decisions of the holy Synods, to wit, of first Ephesus, and of Chalcedon, with regard to the Epistles of Cyril of blessed memory and of Leo, of pious memory, sometime Pope of Old Rome. And since we had learned from these that nothing written by anyone else ought to be received unless it had been proved to agree with the orthodox faith of the holy Fathers, we interrupted our proceedings so as to recite also the definition of the faith which was set forth by the holy council of Chalcedon, so that we might compare the things in the epistle with this decree. And when this was done it was perfectly clear that the contents of the epistle were wholly opposite to those of the definition."
"And since we had learned from these that nothing written by anyone else ought to be received unless it had been proved to agree with the orthodox faith of the holy Fathers" - this is just the Orthodox teaching on the infallibility of the Church; it contradicts as to Roman Catolic teaching (Pope Leo's treatise could not become a rule of faith till it was confirmed by the Council), and to the protestant teachings as well (because of the words "faith of the holy Fathers").
"...And it was shewn in the acts that in former times Ibas had been accused because of the very impiety which is contained in this letter; at first by Proclus, of holy memory, the bishop of Constantinople, and afterwards by Theodosius, of pious memory, and by Flavian, who was ordained bishop in succession to Proclus, who delegated the examination of the matter to Photius, bishop of Tyre, and to Eustathius, bishop of the city of Beyroot. Afterwards the same Ibas, being found guilty, was cast out of his bishopric. Such was the state of the case, how could anyone presume to say that that impious letter was received by the holy council of Chalcedon and that the holy council of Chalcedon agreed with it throughout? Nevertheless in order that they who thus calumniate the holy council of Chalcedon may have no further opportunity of doing so, we ordered to be recited the decisions of the holy Synods, to wit, of first Ephesus, and of Chalcedon, with regard to the Epistles of Cyril of blessed memory and of Leo, of pious memory, sometime Pope of Old Rome. And since we had learned from these that nothing written by anyone else ought to be received unless it had been proved to agree with the orthodox faith of the holy Fathers, we interrupted our proceedings so as to recite also the definition of the faith which was set forth by the holy council of Chalcedon, so that we might compare the things in the epistle with this decree. And when this was done it was perfectly clear that the contents of the epistle were wholly opposite to those of the definition."
"And since we had learned from these that nothing written by anyone else ought to be received unless it had been proved to agree with the orthodox faith of the holy Fathers" - this is just the Orthodox teaching on the infallibility of the Church; it contradicts as to Roman Catolic teaching (Pope Leo's treatise could not become a rule of faith till it was confirmed by the Council), and to the protestant teachings as well (because of the words "faith of the holy Fathers").