First, the basic definition of "heresy", since this is a word that has been used in connection with "subordinationism".

Heresy:

From merriam-webster:

1

• a: adherence to a religious opinion contrary to church dogma
• b: denial of a revealed truth by a baptized member of the Roman Catholic Church
• c: an opinion or doctrine contrary to church dogma

2

• a: dissent or deviation from a dominant theory, opinion, or practice
• b: an opinion, doctrine, or practice contrary to the truth or to generally accepted beliefs or standards

I think it is important to understand that in order to have a "heresy", you have to have an authority to proclaim it OR to have an absolute, definitive theory, opinion, practice or doctrine to which to adhere (and be contrary to).

POINT: If you choose to level an accusation of "heresy", you should have an authoritative church body that represents "the Church" as a whole. If you cannot do this, you are choosing, either as a single person or small body, to proclaim orthodoxy and/or heresy outside of the "body" as a whole.

Accusations of heresy are serious business. False accusations are more serious yet. We should be very cautious when using such words as "heresy" or "blasphemy", lest we bear false witness against brothers and sisters in Christ.

I have little trouble examining doctrine against "historical proclamations" of heresy.  There was a time that was much closer to the cross and the apostles then we are now.  There was a time when the church was united; before the church in Rome and the Eastern Church separated.  Before that time, the young church had several councils that gathered together, examined Scripture and proclaimed "orthodoxy".

So...who decides?

A Church Council is an official ad hoc gathering of representatives to settle Church business. Such Councils are called rarely and are not the same as the regular gatherings of church leaders (synods, etc). An ecumenical council is one at which the whole Church is represented. The three major branches of the Church (Orthodox, Catholic, Protestant) recognize seven ecumenical councils: Nicea (325), Constantinople (381), Ephesus (431), Chalcedon (451), Constantinople II (553), Constantinople III (680), Nicea II (787). Further ecumenical councils were rendered impossible by the widening split between Eastern (Orthodox, Greek-speaking) and Western (Catholic, Latin-speaking) Churches, a split that was rendered official in 1054 and has not yet been healed. (from PBCC.org)

I'll let folks do their own search for the "seven ecumenical councils" - because which of the three major branches will cite different sources, yet all three branches recognize a group of seven councils on which all agree.

In short...if these seven councils agree that a doctrine is "heresy", all three major branches of Christianity today (Roman Catholic, Orthodox, Protestant) will agree.

The Seven Ecumenical Councils:

• The Council of Nicea, 325
• The Council of Constantinople, 381
• The Council of Ephesus, 431
• The Council of Chalcedon, 451
• The Council of Constantinople II, 553
• The Council of Constantinople III, 680
• The Council of Nicea II, 787

To wrap it up, these seven ecumenical councils of the early church met to BOTH unite the church on essential doctrine and to separate those who teach heresy from those who teach truth.

I believe that if a teaching is not found (or condemned) in these seven councils, it becomes more difficult to level an accusation of "heresy". Again, we should be very careful when doing so.