What is heresy? I’ve seen many people use that term for the most trivial reasons, you cannot simply throw around the serious accusation of “heresy” just because you didn’t like what the person said, it must have some sort of historic support and ultimately from Scripture, since infallible Scripture is our final authority. If one cares about theology and heresy they would study theology carefully and Church history to know the development of doctrine of who said what, how others defined what heresy is and who was accused of what so we don’t fall into the same heresy. You’d notice those in the 1800’s are the ones who have committed the most heresies, no surprise, if you don’t learn from the errors of the past you are bound to repeat the errors, these movements rejected the creeds of the past and decided to go their own way, thinking they know better, well they were definitely wrong about that.
You don’t decide yourself what heresy is, heresy is heresy, whether you like it or not
You cannot simply define what is heresy and shift the goal posts when you are close to committing the heresy or to evade heresy yourself, heresy is heresy whether you like it or not, you may be wrong and you would have to deal with it. In saying this you can believe a heresy and not know it, but you’re still in Christ, because one is a heretic if one was to promote heresy and defend it after many tried to warn you about it. Heresy is deviating from the essential or basic doctrines of the historic Christian faith, for an example defined from the councils with support from Scripture which is the final authority. Which doctrines should we avoid committing heresy? Soteriology, Anthropology (study of man) Pelagianism is a noticeable and so is Judaizers , Theology proper (study of God) rejecting God as a Trinity, same with rejecting Christ as divine, or believing the Spirit to be a force. We see the first council in Acts 15 with condemning the Judaizing heresy, that one must be circumcised in order to be saved, that is one must keep Jewish customs this is why the Apostle Paul wrote the epistle to the Galatians.
Who defines what Heresy is?
God. It’s simple as that. Now, all would agree with this answer, but in what ways does God tell us what heresy is? God does tell us in Scripture but through the councils in Church history as well, God works in and through men to defend orthodoxy and refute error. If God didn’t work through these men, how would we know who is right? Now, not all councils are right, they’re not infallible, they can err and contradict each other as Luther stated. God was active in Church history (as He was active in His people in Israel) as Christ stated He will build His Church and the gates of hades will not overcome it Matt 16:18. We cannot simply change the goal posts and decide what heresy is, that’s dangerous for us, I’ve seen that many times, and do terrible research or get their information from the wrong places or authors, because many are anti-Roman Catholic they instantly think anything that is connected to the church is heresy, the Trinity, baptismal regeneration, Divinity of Christ, Mary is the mother of God, Sunday worship etc… there are groups of people that would condemn those views because of Rome.
Believing the creeds is not strange to the Reformed. The Swiss reformer Heinrich Bullinger stated this in his Second Helvetic Confession chapter 11
The Creeds of Four Councils Received. And, to say many things with a few words, with a sincere heart we believe, and freely confess with open mouth, whatever things are defined from the Holy Scriptures concerning the mystery of the incarnation of our Lord Jesus Christ, and are summed up in the Creeds and decrees of the first four most excellent synods convened at Nicaea, Constantinople, Ephesus and Chalcedon–together with the creed of blessed Athananasius, and all similar symbols; and we condemn anything contrary to these.
Learn from the past will prevent you from repeating the same heresies
We’ve heard the famous statement by Winston Churchill many times before “those who forget history are doomed to repeat it” that’s exactly the reason why we should study Church history otherwise how else would we know which heresies we should avoid? Because if you don’t know which heresies to avoid you might be believing the heretical teaching itself, which no one in their right mind wants to do. Not only is studying Church history beneficial to avoid heresy but also to believe orthodox (biblical) teaching, how should we explain the Trinity? The Athanasian creed on the Trinity is exceptional to understand how to and how not to explain the Trinity in detail.
So, in a short summary, we don’t need to exactly think how we should explain the Trinity or the hypostatic union (the two natures) of Christ, when the hard work has already been done for us that is by the Christians who went before us who studied these doctrines deeply. If we reject studying Church history we will eventually fall into heresy sooner or later.