I write considering a definition of heresy, given by Grosseteste, Bishop of Lincoln (fl. 1235), who's words are widely cited in regards to the history of the inquisition:
An opinion created by human reason. founded on the Scriptures, contrary to the teachings of Christ, publicly avowed and obstinately held.
You will find this definition often cited in both scholarly and popular works on the history of the Inquisition (which, incidentally, Grossetesse wasn't part of.)
It is an accurate definition, too.
However, almost every source that I find using that English cites either nothing, other English works, or Paris's Chronica maiora (ca. 1250), in particularly, the authoritative version of Henry Luard of 1872. All well and good. The context there is only moderately illuminating, but rather alarmingly it has Grosseteste saying something rather different, and more than a little significantly so--
Heresias est sententia humano sensu electa, Scriptura Sacrae contraria, palam edocta, pertinaciter defensa.
(This is the primary source. Roughly, "heresy is an idea arrived at by human reason [that is] contrary to Sacred Scripture, taught openly and obstinately defended." Similar in form but quite different, although not exactly contradictory, in meaning.)
I suspect that the English sources are circularly quoting one another, rather than bothering to read their Latin; why bother, I'd imagine? I probably wouldn't have if not for my current specific interest in this specific quote. But the quote is quite everywhere, particularly in sources dealing the early Inquisition. And the distinction is very much one with a difference.
A sententia...Scritupra Sacrae contrariae versus ex/derivate a/etc. Scriptura Sacrae are clearly two different things. Most significantly, the latter allows for the fact that non-Christians are not heretics, and that heresy is only a misinterpretation of Christianity, but the former would subsume all non-Christian thought into the idea of "heresy" (which is not what is generally meant or has historically meant to be the case.)
The English translation I can't source seems to provide a better definition than the Latin original than I can, cf. Matthew Paris --- but what's going on here? Is there a different quote from Grossetesse that I have missed? (Feels doubtful.) Has someone editorialized a definition on top of Grossetesse's? Probably a better one, certainly in line with the later definition of Aquinas and the eventual Catechism, see below: Jews and Muslims are not heretics, although some have called Islam a heretical offshoot of Christianity, notably the late-19th c. Reformed scholar Schaff, but he wasn't very familiar with it beyond secondary sources at best. In today's religious landscape, for instance, the Arianistic Jehovah's Witnesses would be heretics, founding their heresy, as they do, on Scripture; the Mormons are arguably not because they are an entirely different religion with their own texts and teachings even if given a Christian gloss.
Context: Grossetesse was too early and too English to be involved with the Holy Office, but he was a highly influential churchman and perhaps one of the smartest men in England in the time. One of his priorities was church reform, and one of his ideas was to establish a (small-'i') "inquisition" into the Church in England, as far as clergy education, abuses, heresies, etc.
Any thoughts would be most welcome.