Why is rejection of the Trinity heresy, and not just wrong? (Protestant perspective) There are plenty of instances where Christians systematically believe something false and directly contradicting biblical teachings, yet they are not deemed to be so errant as to disqualify them from salvation and from fellowship with the universal Church.

By way of example, predestination unto salvation is a mutually exclusive belief about the nature of God such that some Christians directly deny it and others directly confirm it, yet neither believes the other to be heretical.

Yet it is almost universally held in Trinitarian Christianity, that denial of the Trinity is a renunciation of Christianity and salvation.

Why? Why isn't it simply, wrong? Is it mainly due to the early historical discernment of the early councils, or is there a better, biblical argument for this case?

I'm coming from a Protestant perspective. Certainly, a Catholic perspective could get away with saying, "The Church has denounced it." For a Protestant, what does it take before something is properly known as a heresy?

relevant: What is &quot;heresy&quot;?
A famous quote from Christian history is:


  In Essentials Unity, In Non-Essentials Liberty, In All Things Charity


In Essentials Unity

The Apostle Paul talks about "another gospel" that was being preached to the Galatians, and he uses very strong words regarding those who bring such a gospel:


  6 I am amazed that you are so quickly deserting Him who called you
  by the grace of Christ, for a different gospel; 7 which is really
  not another; only there are some who are disturbing you and want to
  distort the gospel of Christ. 8 But even if we, or an angel from
  heaven, should preach to you a gospel contrary to what we have
  preached to you, he is to be accursed! 9 As we have said before, so
  I say again now, if any man is preaching to you a gospel contrary
  to what you received, he is to be accursed!  Galatians 1:6-9 NAS


The concept is that there is one true gospel that leads to eternal life.  As Jesus said in Matthew 7, there is one narrow road that leads to life and one wide road that leads to destruction.

So, we only have unity with those who hold to the essentials of true gospel of Jesus Christ.  To put it another way, we only have unity among those with whom we will spend eternity.  We do not have unity with those who preach another gospel that does not lead to life.  We have charity, but not unity.

So, the question becomes centered on what are the essentials.  From an evangelical perspective, the essentials typically include the nature of God (including the Trinity) and salvation by grace alone through faith alone in Christ alone.  Thus, whatever departs from that is considered heretical.

John's first epistle speaks about the necessity of correct Christology and Theology:


  I have not written to you because you do not know the truth, but
  because you do know it, and because no lie is of the truth. 22 Who
  is the liar but the one who denies that Jesus is the Christ? This
  is the antichrist, the one who denies the Father and the Son. 23
  Whoever denies the Son does not have the Father; the one who confesses
  the Son has the Father also.  1 John 2:21-23


Specifically, the "liar" is the one who denies that Jesus is the Christ.  The antichrist is the one who denies the Father and the Son.  This is understood by many to deny the nature of Jesus as the eternal Second Person of the Trinity.

Additionally, John also writes that anyone who denies Jesus as having come in the flesh is antichrist:


  For many deceivers have gone out into the world, those who do not
  acknowledge Jesus Christ as coming in the flesh. This is the deceiver
  and the antichrist.  2 John 1:7 NAS


In Non-Essentials Liberty, All Things Charity

We may disagree with others about election or the end times, and there is freedom in that.  I personally disagree with my pastor about a few things, primarily, but not limited to eschatology.  Yet we still have unity.

I am exhorted to oppose those who hold beliefs I consider heretical, but I do so with charity.