“Heresy” is a term that is thrown about far too easily. Some defend the use of the word as denoting some belief or teaching that is not in the Bible, or contradicted by the Bible. Essentially, this is any belief that is rendered incorrect by another belief about Scripture. So what’s the problem? Well, if this is how heresy is defined, then one of three results can occur.
First, you can insist that everyone around you hold precisely the same set of beliefs as you do, and find yourself with no one to be around. This is because virtually no two free Christians find themselves holding a 100% match with someone else. Pry long enough, and there will eventually be some disagreement. But on the definition of heresy, you should regard those who disagree with you as being incorrect. Presumably, you will regard them as incorrect, or else you should join them! But then everyone else will be a heretic. Second, you can insist that everyone around you hold precisely the same set of beliefs as you do, and find yourself a cult leader (or member, if you look in the right places and adjust your own beliefs accordingly). But that’s not good.
Finally, you can divest “heresy” of most of its force. That is, you can take this definition and apply to it anything you disagree with, and anyone who disagrees with you is a heretic (since, after all, you wouldn’t believe it if you thought it was false!). Does the verse that says “the love of Christ constraineth us” mean Christ’s love constrains us or our love for Christ constrains us? If you take a position, then the other one is heresy for you. But most of us recognize it would be crazy to break fellowship over such a thing, so “heresy” and “heretic” just don’t express more than “I disagree with this person.”
So these are your choices when you define “heresy” so poorly. Either you are isolated, join a cult, or heresy means nothing. None of these options is good. Instead, “heresy” should be reserved mostly, if not exclusively, for those beliefs, the embracing of which means one is not a Christian. This way, we can finally cut off calling Calvinists, Arminians, Molinists, and most all Christians “heretics.”