What I'm about to share could have also been titled "What Constitutes Heresy?" because there's a difference between core essentials, differences of opinions, and differences between the levels of maturity of believers. There are far fewer heretics in the Christian Body than most Christians would imagine. The New Testament tells us what heresy is: a denial that Jesus is the burden remover, yoke destroyer (Isaiah 10:27; 1John 5:1), a denial that Jesus is the Son of God (1John 5:10), a denial that Jesus came in the flesh in His birth and in His resurrection (1John 4:2-3). Heresy is also practicing those things that the Bible clearly says will cause one to never inherit the Kingdom of God under any imaginable coming dispensation. Inheriting the Kingdom is different from having one's sin removed by the Blood of God's Son and being reconciled to God. 1Corinthians 6:9-10 and Galatians 5:19-21 list things that prevent someone from inheriting the Kingdom of God. Violations of the commandment of love on the part of individuals are damnable heresy. But institutions according to Romans 13 don't bare the sword in vain and aren't in the same category in Scripture as individual responsibility because the institutions are responsible for the care of the many in this life. 2John 1:9 says that those who don't continue in the teaching of the Anointed One and His Anointing don't have God. 1Corinthians 3:17 specifies that those that are disrupting a fellowship of believers are on the chopping block because God's going to disrupt them. 1Timothy 1 specifies those that lack self control as being contrary to sound doctrine.
And then there's the things that the New Testament speaks to those that are growing in Christ. There's a way of reading the whole New Testament, and many people won't get this their first half dozen readings of it, but as you're maturing in grace you'll come to the place where being in financial debt is abiding in false doctrine. There's other areas where the Word will get plainer with the individual as they mature in the Anointing and having it flow through them that the Bible won't allow them to participate in. Such as when a minister is going to minister the Word to others, you reach a point where getting chatty before you're going to deliver the Word to people grieves the Holy Spirit and costs you [very deeply] spiritually, so that when you're entering into a ministry opportunity, you're really supposed to be sanctified from the people before and for a short time after you've shared what God's given you to share. I know that those that have a problem with Apostolic, Prophetic, and really Anointed Teaching gifts still being in the Body per Ephesians 4 will have a problem with what I've just said, but I'm talking about an area in this second paragraph that's for the maturing and not the common ground areas where you can walk into heresy that I mentioned in the first paragraph of this.
As there's many levels of truth, there's many levels of heresy, but the mature shouldn't throw out the immature as being heretics if they're able to abide by the very obvious things that I covered in the first paragraph of this. Things that would send me straight to Hell (and I'm choosing my words advisedly), for many people God's never even talked to them about those things for various reasons and they're not accountable for that yet. And that doesn't mean that I'm all of that particularly important in the Body of Christ. It just means that I've had very clear instructions from our Lord about certain things that I'm morally accountable for. Each of us living according to their own convictions about this and that was a constant theme of St. Paul's when it came to things that weren't immediate doctrinal essentials that could cost an outsider their soul for the ages of ages. Tampering with the rightful place of the Cross, the Blood, the resurrection, the ascension, the second coming, the commandment of love, the commandment to be responsible to authority, the commandments to maintain good fruit and personal purity, and the commandment to disciple the nations gets you into damnable heresy, not so much over your own damnation although that would be bad enough, but because of the harm that you could bring to others and their salvation within the ages of ages.
And contrary to what some have said in the past, that I've personally heard or read something from, if there is no Return of Christ Jesus, then all that the first disciples were told about the resurrection and ascension of Christ Jesus is a total lie because of Acts 1:11. The Return of Christ is a cardinal doctrine of the New Testament, meaning that it's basically a deal maker/breaker. It's as essential to Christian doctrine as any of the 4 spiritual law pamphlets you might bump into somewhere about having been created, something having gone horribly wrong, Christ being the solution, and the obligation of each of us to repent and believe the Gospel according to St. Paul's teachings that Christ's Blood effectively and permanently removes our sins.
Basically, the gist of what I've just said is that what's costly about heresy is how much we wind up costing someone else with what we share with them. That's the whole point of the really strong statements in the New Testament about this. Is what we're sharing something that could cost someone their salvation and sanctification for the ages of ages? It's not about being on a perfectionist kick, but the sober reality that we can directly influence someone to be shut out from His Presence for the ages of ages. But if we avoid speculations and simply keep it to the Book, then there aren't any dangers. For Christians who fear over-literalization, the only potential danger to their methods [by keeping it literal where applicable] is in terms of growth and breadth of outlook, but at least no one did anything stupid on the absolutes by keeping it "line upon line, precept upon precept." The Book's true. What we get out of it needs to be scrutinized so that out of the mouth of two or three witnesses...