Scot McKnight has a very handy guide to when to use—or refrain from using—the “H” word.
Let me suggest that the term “heretic” is used in three ways, only one of which (I believe) is justifiable — though I have little hope that the mudslingers will learn to use terms as they are supposed to be used.
Before I get there, though, let me add another point: it is too bad we don’t have such an evocative term for praxis. Jesus’ focus was on “hypocrisy” more than “heresy,” and it might just be an indication of how far we’ve strayed for us to give so much attention to “heresy” and not enough to failure in praxis. As far as we can see, failure in practice is just as bad as failure in theology. But this is not what this post is about. We are concerned here with the term “heretic.”