Heresy is a set of opinions "at variance with established or generally received principles." In this sense, heresy is the price of all originality and innovation.

Heresy is the eternal dawn, the morning star, the glittering herald of the day. Heresy is the last and best thought. It is the perpetual New World, the unknown sea, toward which the brave all sail. It is the eternal horizon of progress. Heresy extends the hospitalities of the brain to a new thought. Heresy is a cradle; orthodoxy, a coffin.

It is on account of theology alone that any assertion whatsoever should be called catholic or heretical. For only an assertion which is consonant with theology is truly catholic, and only one which is known to be opposed to theology is known to be heretical. For if some assertion were found to be opposed to decrees of the highest pontiffs, or also of general councils or also to laws of the emperors, nevertheless, if it were not in conflict with theology, even if it could be considered false, erroneous or unjust, it should not be counted as a heresy.

Heresy is the eternal dawn, the morning star, the glittering herald of the day. Heresy is the last and best thought. It is the perpetual New World, the unknown sea, toward which the brave all sail. It is the eternal horizon of progress.
Heresy extends the hospitalities of the brain to a new thought.
Heresy is a cradle; orthodoxy, a coffin.

Whoever has an opinion of his own, and honestly expresses it, will be guilty of heresy. Heresy is what the minority believe; it is the name given by the powerful to the doctrine of the weak.

The heretic is one who has an opinion; and that is what the word itself signifies. What does it mean to have an opinion? It is to follow one's own particular thought and feeling. But the Catholic is catholic: that is to say, he is universal; and without having any particular sentiment, he follows without hesitation that of the Church.

When a heretic wishes to avoid martyrdom he speaks of "Orthodoxy, True and False" and demonstrates that the True is his heresy.

… we sometimes find that such heresies have been the foundation for bold and necessary change, but heresy is usually just crazy. Most daring new ideas are foolish or dangerous and appropriately rejected or ignored. So while it may be true that great geniuses are usually heretics, heretics are rarely great geniuses.

The heresies we should fear are those which can be confused with orthodoxy.

Although certain to be contradicted by every pious churchman, a heretic must insist on thinking that the Mater Dolorosa was the logical Virgin of the Church, and that the Trinity would never have raised her from the foot of the Cross, had not the Virgin of Majesty been imposed, by necessity and public unanimity, on a creed which was meant to be complete without her.

Could you punish a heretic?"
"Madam, I do not know what a heretic really is. The word is made much of, these days. It seems to describe anyone you wish dead.

We have never said that the Council directly professed heresies. But the wall of protection against error was removed, and thus error was allowed to manifest itself. The faithful need protection. This is the constant struggle of the militant Church to defend the faith.

It is easy to see what is heresy today, but who can tell what will be heresy tomorrow

The Pope may err, and that in two ways, either because he is erroneously informed, or from malice. As to the latter cause we leave that to the judgment of God, and believe rather that he has been misinformed. In our own case I can prove that he has been falsely persuaded. Therefore any one who obstinately upholds the excommunication and affirms that I ought not to preach these doctrines is fighting against the kingdom of Christ, and supporting the kingdom of Satan, and is himself a heretic, and deserves to be excluded from the Christian community.