Try as some (just Ohio I think

) might, to claim that I use some sort of fallacy to explain the term heresy, the fact is I have only used the Bible and the
Bible alone to define the term.
The way the Bible defines heresy is I believe the way God defines heresy, and not as man has defined heresy (in relation to man-made Creeds, Confessions etc).
In short, the Bible defines heresy as denial of who Christ was and what Christ did (the gospel), and those who stir up divisions in relation to the denial of Christ.
The early church faced two main types of heresy - gnosticism and Judaism. Gnosticism was heretical mainly because of its denial of the person of Christ - that Christ did not come in the flesh. The book of 1 John combats this heresy. Judaism was heretical mainly because of its denial of what Christ did on the cross - by claiming that grace through faith alone was insufficient. The book of Galatians combats this heresy. Gnosticism was more than just a theological threat - the production of many "gnostic gospels" such as gospel of Judas etc could well have derailed the church if not for the works of the early church fathers. The church fathers themselves, had a variety of strange opinions and ideas themselves, but they weren't considered heretics because they held to the teaching of the apostles regarding the person and work of Christ. For example, some believed in infant baptism, which some Christians today may consider heretical. We should remember that the Creeds were written to combat these serious threats to the Church, and not meant to be a "catch-all". We can see that none of these early church concerns pertain to a theological argument about the Trinity. People who "disagree with the Trinity" does not even rate a mention in the Bible, meaning it was not seen as important by the Apostles.
A complete list of verses pertaining to heresy is found here:
https://www.openbible.info/topics/heresy
A quick review of these verses show clearly that heresy is defined as:
1) denial of Christ in the flesh, another Jesus, or another gospel (2 Peter 2:1-22 , Galatians 1:8-9etc). - this is heresy by denying the person of Christ or the gospel which pertains to the work of Christ.
2) Wicked works of the flesh, (2 John 1:11, Gal 5:19)
3) Division and rebellion (1 Samuel 15:23 and verses I quoted in previous post). This denotes divisions or schisms in "the church".
Points 1) and 2) should be obvious as to their application - anyone denying Christ and claiming to be a Christian is a heretic.
Point 3) is trickier because it is coupled with our view of "the church". The misapplication of point 3) by Catholics, Protestants and Reformers was to claim to be "the church" and thereby claim that those who disagree with them are heretics. In this way, Catholics viewed Luther as heretic, Luther viewed Catholics as heretic, and Church of England viewed both as heretic (a rather entertaining history and origin with King Henry VIII etc). There was also some heresy dynamic going on between Calvinists and others.
It does not make much sense that any group can claim to be "the church" and claim others to be heretics. If that were the case, I could start a new church tomorrow and claim others are heretics if they do not accept my new church's Creed or Confession. The definition of heresy in relation to the Creeds or the Trinity etc is more strictly,
theological heresy and the Bible is relatively silent about that. It is not biblical heresy and in God's eyes maybe isn't heresy at all. God is mainly concerned with what we believe about Christ.
Some, may think of the Reformation as a noble act of God, a single, well defined event that gave us Christianity in its purest form. However the reality was that the Reformation was a mess, a flurry of activity as people tried to break from the Catholic church, complicated by the involvement of governments, politics, and kings (King Henry etc). None of these groups decided to sit down together and agree on the biblical definition of heresy - they were too busy calling each other heretics and killing them for it.
I personally believe that during the time of Constantine, genuine believers were outcast or even killed as heretics simply for not agreeing with the dogmatic definition of the nature of God, known as the Trinity or the Nicene Creed, as defined. While the Creeds served their purpose in resolving obvious heresies such as gnosticism and Judaism which deny the person or work of Christ, I believe the early church had a wide variety of opinions about the nature of God which may not have disqualifed them as heretics according to the biblical definition (points 1),2),3) above).