>>10912 Is correct in saying heresy is false teaching so your question can be restated as "Whence true teaching?" It's obvious that anyone's answer to this is going to be guessable from what kind of church they go to. The more interesting question is where you draw the line between Christian and non-Christian. This seems like it has an even more trite answer - my church is True Christianity and everyone else is not - but, in practice, most churches will refer to most other churches as Christian. Often it comes with qualifiers but the word "Christian" is still used. At the other end, there are some self-professed Christian groups for whom it is uncontroversial to exclude from Christendom. Mormons, for example, prosperity gospel preachers, and Seventh Day Adventists among others. I think if you oppose the seven ecumenical councils or any of the three creeds or else if you so distort the faith that it's unrecognisable as the faith that has been practiced by the majority of Christians for the last 2000 years, then that must surely put you outside the bounds of Christendom. I understand this last criterion is a little vague but it's necessary to exclude people who gut the faith of things that are obviously necessary to hold to for Christians but were not specifically addressed in the councils or creeds. An example would be if you denied our need for repentance and forgiveness. At that point you've obliterated the faith by making the incarnation, crucifixion and resurrection pointless. Heresy would be any false teaching from within Christendom that doesn't rise to the level of apostasy (a renunciation of the gospel).