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 "heresy"?What is "heresy"?
