A question about the definition of heresy (13th. C)

There was no one proper or good or better definition of heresy at the time, except perhaps by raising that a heresy in those days means whatever the Catholic Church or its representative on the ground deemed inadequate, with the added twists that the Pope's primacy over the Catholic Church wasn't recognized until well after the fall of the Western Roman Empire, and in some cases a King might step in to get the finger pointed at some inconvenient group. Take for instance the Knight Templars, who got accused of heresy so the King of France could pocket their wealth. (They cleared their name recently.)

To put Grossetesse's thoughts in more context, Wikipedia has a helpful -- and very long -- list of heresies according to the Catholic Church that gives a flavor of the kind of intense theological debates that went on in the Medieval era (and later). The theological debates at the time were many and raging, and it was not unheard of to have rulers that ended up considered heretics. The Lombards, for instance, supported the Arianism heresy. Also, and in addition to the more famous crusades against the Saracens, there had been plenty of crusades against Christians -- including some in the 13th century. Sometimes heresies were purely theological in nature, but there also are examples (such as the Templars mentioned further up) where it was purely realpolitik at play.

Another point to keep in mind is that the Catholic Church at the time did not in any way resemble what it looks like today. The Papal State was a powerful country. More so even by virtue of having branches in other countries. Its thin veil of religiosity gave it a lot of clout. But don't make the mistake of thinking it was the haven of morality that it tries to pass off as today. (It miserably fails to be, I should add, in the event anyone forgot that a notorious micromanager was in charge of problems such as pedophile priests before he took the helm). One of the worst ever popes might have been John XII. He reigned in the mid 10th century, and basically turned St Peters into a brothel for European Elites. He also reportedly raped hundreds of nuns and pilgrims, and, in an effort to remain in power when he shocked Otto I, he ended up threatening to excommunicate anyone who attempted to depose him.

I do not have a answer about your quote, just an issue about the definition which may help or not.

What I find strange in your quote is that it is not clear that the heretic must have been catholic before, or at least believe generally in Christ. Jews also may have beliefs or opinions based on or derived from Scripture, but they can not be heretics.

St Thomas' Summa has its own definition (from the same century), besides a more detailed explanation than your small quote.

Therefore heresy is a species of unbelief, belonging to those who profess the Christian faith, but corrupt its dogmas.

http://www.newadvent.org/summa/3011.htm

The modern definition also looks similar (from CCC). note the word post-baptismal

2089 Incredulity is the neglect of revealed truth or the willful refusal to assent to it. "Heresy is the obstinate post-baptismal denial of some truth which must be believed with divine and catholic faith, or it is likewise an obstinate doubt concerning the same; apostasy is the total repudiation of the Christian faith; schism is the refusal of submission to the Roman Pontiff or of communion with the members of the Church subject to him."11

Also, both St Thomas and the modern CCC make clearer that the error must be about important matters of faith. The expressions "dogmas" or the modern formula "truths that must be believed with..." look more precise than just generically stating "the teachings of Christ"

In short, I have 2 reasons to prefer St. Thomas' definition.

It is possible that your quote has an immediate context where its meaning is made clearer, just as it is with St Thomas?

And, are you sure your quoted definition was really important at its time? I would guess St Thomas was more relevant.