Heresy is to disagree with the church. Which church should I accept as the authority of what is heresy?
Discussion
That's a good question.
To drastically oversimplify it ("the medium affects the message"):
The whole church embraced the Apostle's Creed, so start there. For what is meant by it, see the Nicene Creed, Athanasian "creed", and the definition of Chalcedon. This means orthodox (with a small 'o') Trinitarianism.
From there, testing everything according to Scripture, decide who continued the Apostle's teaching, and *on what grounds*, whether Rome or the Reformers.
Then, decide your position on the Synod of Dordt.
There hasn't been much new by way of "heresy" since then--just periodic revivals of questions long since answered.
Shortcut: embrace the Westminster Standards, the Three Forms of Unity, or (if you reject household baptism) the 1689 LBCF. And you'll generally be in a good spot in terms of where the Apostle's teaching persists today.
*Westminster (American Revision of 1788)
I accept your argumentum ab auctoritate regarding the definition of christianity.
Sola Scriptura above all.
The question is, which groups have remained most faithful to the teaching of Scripture (Acts 17:11) -- despite any self-referential claim to authority?
As a Protestant I affirm the "succession of truth" over the so-called "Apostolic Succession" of Rome -- for the simple reason that God's Word always takes the place of preeminence over man's word.
Hence: sola scriptura
I have no qualms with sola scriptura. I also have no contention with the fruit of the tree argument. As a reformed catholic myself, we are on the same side there. The only difference is in the interepretation of the word. Knowing what I have learned from this interepretation and hence the consequence of this knowing that leads me to very different conclusions than what the church has taught since Constantine and the Council of Nicea.
Sola scriptura is a fallacy from a mind that made up a lot of new formulas ignoring fundamental theological aspects.
The tradition came first.
If you did believed scriptures came first, you would have never accepted the removal of 7 books, from the original 73 to 66 of the protestant imposters.
How do you explain their removal without attacking the scriptures themselves?
We received the OT from the church under age. The "aprocrypha" were never considered the Word of God, even if helpful or historical (or not). See Kline, [The Structure of Biblical Authority](https://amzn.to/3rz8MMj). Either way, authority of an inspired text is not bestowed (per Rome) but recognized and acknowledged (as per the Reformers). God's word speaks for itself.
In our circles, this is reffered to as [the self-attestation of scripture](https://modernreformation.org/resource-library/articles/the-self-attestation-of-scripture/) (q.v.).
Some books, even of the 66, are more important than others.