The term "heresy" is seen in our times as an odious term, a term of bigotry and intolerance. Even in professing Christian circles, some like Alister McGrath see the term as limited in scope referring to errors concerning certain views of God and Christ in the early church which were declared "heretical" in church councils. In other words, "heresy" is a formal term; an ecclesiastical term. Under such a definition, only errors like Sabellianism or Monophysitism are considered heresies. Other errors like Socinianism and Liberalism are presumably not "heresies" since no ecumenical council has ever declared them heresies.
I am strongly against such a historical definition of heresy. Of course, such a definition is "helpful" in the sense that it makes the category of "heresy" easier to manage, and is congruent with the ecumenical spirit of the age. But it also relegates "heresy" as a past historical artifact, something which was historically decided and of which the present church has nothing to add. The question then is: What has changed from the past to the present? Did the church change such that she could declare certain false teachings as heresies in the past, whereas she somehow loses the authority and power to do so today? What exactly has changed?
Furthermore, the implications of making "heresy" merely an ecclesiastical term lead us to some rather strange conclusions. Prior to Nicea, were the Arians orthodox and non-heretical, since the Council of Nicea (325AD) has not yet commenced? If the claim is made that "heresy" is an ecclesiastical term, then the answer given should logically be yes. Such a view should entertain the strange notion that the Arians were orthodox before the Council of Nicea, and heretics after the Council. In other words, their statuses were changed at the council itself. And since heretics cannot be saved, does that mean that an Arian dying before the convening of the Council could expect to go to heaven, while an Arian dying after the conclusion of the Council could expect an eternity in hell?
These are the strange conclusions that those who advocate for defining "heresy" as a historical and ecclesiastical term should embrace. I guess they could choose to bite the bullet, but it seems to me such strange conclusions is an indication that defining "heresy" so narrowly is wrong.
If instead we define "heresy" as serious doctrinal errors that imperil the salvation of those who believe it, then it seems that we have a definition that does not leave us with such strange conclusions. Of course, that means that defining what is and what isn't heresy is going to be a messy business, but I think that such messiness is preferable to the strange conclusions that follow from using the formal, ecclesiastical definition.