Heresy and orthodoxy
Heresy is first of all a term of church discipline, not false belief. It is the application by duly constituted ecclesial authority of a certain status to persons, groups, movements, practices, or ideas. That status is anathema: the curse of excommunication. "Such entities do not belong in the community of Christ" is what heresy announces, publicly and definitively. If the relevant persons or groups do not thereupon repent, they become "heretics" or "heretical," which is a formal status relative to a concrete Christian community or communion. Subsequent to the decisive events that constitute said entities as "heretical," similar ideas and practices might be judged at the popular level to be of a piece with that which was previously anathematized; thus ordinary people might label a notion "heretical" by derivation from prior authoritative pronouncement. But heresy as such remains a matter of church discipline. It isn't something one makes up oneself, much less promulgates on one's own.
Thus understood, heresy is the shadow side of orthodoxy. Orthodoxy is the positive dogmatic teaching of the historic church, that is to say, of the historic episcopal and conciliar church. You do not have to wonder what orthodoxy teaches. Read the canons, creeds, definitions, and anathemas of the seven ecumenical councils, alongside and as an interpretation of Holy Scripture. That is orthodoxy. That, at least, is what the term "orthodoxy" means, if it is to have any substantive or historically coherent meaning at all. It may mean more than that, given the divided state of the church in the last millennium, but it does not and cannot mean less.
I trust that goes some way toward explaining my dissatisfaction and confusion with conventional low-church Protestant and evangelical usage of these terms. For such usage, the terms not only come to be defined wholly relatively—in terms of this or that sub-sub-group new on the ecclesial scene—they also assume meanings contrary to their original historical and dogmatic sense. So that, for example, "heresy" names the veneration of icons, or the mandatory celibacy of bishops, or having bishops in the first place, or the acclamation of Mary as Theotokos. But this is gibberish. One may certainly make arguments that such teaching and practice are wrong, or unwarranted by biblical precedent. But to call them "heresy" is to invert the term's historic sense. Moreover, it is to ignore the word's original and abiding force as an expression of churchly authority, and to deploy it instead as meaning merely "something with which I/we disagree."
"Orthodoxy" fares no better. In popular usage in the last century—popular, that is, among the non-catholic groups I've identified above, but used broadly by their scholars, pastors, and theologians—"orthodoxy" comes to mean "those parts of the historic tradition we continue to affirm, minus those parts we do not." But you can't have your cake and eat it too. Such an operation is surgical. It slices and dices, cutting off elements of the tradition that the majority church (both globally and historically) once thought and still thinks essential. The result is something of a Frankenstein's monster, only without the admission that it is man-made. But the so-called "historic" or "traditional" "orthodoxy" thus proffered and appealed to as "common" and "ancient" is self-evidently an artifice. Perhaps it is true artifice—perhaps it cuts through those other man-made traditions that, like so many weeds, grew up around the gospel, threatening to overwhelm it—but that is a different claim than calling it "orthodox." To do the latter one aspires to participation in and affirmation of the ancient patristic (and perhaps medieval) articulations of the faith. But in lopping off one-third of those articulations while revising or amending another third, one undercuts the perceived benefit of appealing to ancient tradition. If the dogmatic inheritance is revisable, but one's appeal to its antiquity and unanimity is meant to shore up its unrevisability, the internal contradiction should be obvious. You can't have it both ways.
This is all a very long way of saying that division in the church matters. Such division is not only real, it is rooted in and gives rise to opposed doctrines and practices. There is, in other words, a logic to the mutual anathemas of the 16th century. If we teach X and y'all teach not-X, either one of us is right and the other wrong, or both of us are wrong. It can't be the case that both of us are right (at least if we understand what we are saying). So far as I can tell, we want today to be able to affirm a deep commonality across church division: and that is a good desire. But it ought to be grounded in the truth. Some church traditions can affirm what I outlined above as the bare minimum of historic orthodoxy. It seems to me that those traditions have a good deal of common ground on which to talk about their differences. Other traditions, less so. That doesn't mean the ecumenical task is dead on arrival. Nor does it mean per se that the "orthodox" churches are eo ipso right and therefore those that cannot claim orthodoxy are in the wrong. It simply means that we should use these terms with care, so that they have discernible content. It also means that those traditions that have departed from historic orthodoxy ought to admit the fact; ought to stop using the term; and certainly ought to quit the rhetorical habit of laundering the purity of their doctrines through a misplaced appellation of a venerable ancient term.
I'll let Inigo Montoya have the last word: "You keep using that word. I do not think it means what you think it means."