Decree Sacrosancta of the Council of Constance, Fourth Session, 1415
(approved by Pope Gregory XII)
And since by divine right of Apostolic primacy the Roman Pontiff is placed over the universal Church, we further teach and declare that he is the supreme judge of the faithful, and that in all causes, the decision of which belongs to the Church, recourse may be had to his tribunal, and that none may re-open the judgment of the Apostolic See, than whose authority there is no greater, nor can any lawfully review its judgment. Wherefore they err from the right course who assert that it is lawful to appeal from the judgments of the Roman Pontiffs to an ecumenical Council, as to an authority higher than that of the Roman Pontiff.
Session IV (ch. iii) of the First Vatican Council, 1870
Then, moreover, what a swelling of arrogance it is, what oblivion of humility and gentleness, what a boasting of his own arrogance, that any one should either dare, or think that he is able to do what the Lord did not even grant to the apostles; that he should think that he can discern the tares from the wheat, or, as if it were granted him to bear the fan and to purge the threshing-floor.
But I beseech your imperial piety to consider that some frivolous things are very harmless, and others exceedingly harmful. Is it not the case that, when Antichrist comes and calls himself God, it will be very frivolous, and yet exceedingly pernicious? If we regard the quantity of the language used, there are but a few syllables; but if the weight of the wrong, there is universal disaster. Now I confidently say that whosoever calls himself, or desires to be called, Universal Priest, is in his elation the precursor of Antichrist, because he proudly puts himself above all others.
Pope Gregory the Great
There is no other head of the church but the Lord Jesus Christ: nor can the Pope of Rome in any sense be head thereof; but is that antichrist, that man of sin, and son of perdition, that exalteth himself in the church against Christ, and all that is called God.
Let no one cheat himself, let no one deceive himself. The Lord alone can have mercy. He alone can bestow pardon for sins which have been committed against Himself, who bore our sins, who sorrowed for us, whom God delivered up for our sins. Man cannot be greater than God, nor can a servant remit or forego by his indulgence what has been committed by a greater crime against the Lord. . . .”
[W]e define that there shall be expelled from the Holy Church of God and anathematized Honorius who was some time Pope of Old Rome, because of what we found written by him to Sergius, that in all respects he followed his view and confirmed his impious doctrines.
The Sixth Ecumenical Council, 680
For the Holy Spirit was. . . promised to the successors of Peter. . . that by his assistance they might inviolably keep and faithfully expound the revelation or deposit of faith delivered through the Apostles. And, indeed, all the venerable Fathers have embraced, and the holy orthodox doctors have venerated and followed, their apostolic doctrine; knowing most fully that this See of holy Peter remains ever free from all blemish of error. . . .
First Vatican Council 1870