Disputes over theological heresies have plagued Christianity since early times.
Sometimes heresies are lethal, sometimes they seem trivial, sometimes they are very significant in terms of what theology implies about the underlying motivations.
For example, my interpretation of the Filioque dispute which divided the Western and Eastern Churches, is that the Eastern objection was to the act of revising the creed (on whatever grounds - even when, as the Western Church believed was happening, they were simply making it more accurate), rather than to the true theological implications of the Filioque (which are incomprehensible to all but very few).
Because theology is almost wholly obscure to almost everybody. C.S Lewis tried never to write anything about theology, and tried not to discuss it, because he considered it too high a matter; and except among the most learned and holy disputants, that such discussions led to arguments led to schisms and weakened the Church.
At what I regard as the height of Christian society, within the Byzantine Empire, there were several major theological disputes. One was the 'Monophysite' dispute starting in the AD 400s, an outcome of which was the separation of several North African Orthodox churches including the Coptic church in Egypt. This separation remains.
The point at issue was how to describe the nature of Christ in terms of God and Man - for example, whether He was God and Man separate, or fused, or the one absorbing the other.
Now, although I can read and use the words of this dispute; this is a matter far beyond my real comprehension. I am happy to accept the teaching I have been brought up on; but if I had had another kind of teaching or emphasis or form of words - no doubt the same would apply.
And one and a half thousand years of history have, I think, shown that this was a theologically trivial dispute - since the separated Orthodox Churches (such as the Coptic) have certainly remained Christian by any sane definition; have indeed survived extraordinary persecutions.
(Although this situation has now successfully been destabilized, and it seems possible that we may soon see the end of the Coptic church in Egypt).
Presumably there is a correct answer to the Monophysite dispute, but it seems (from what we can perceive) that it doesn't (so far as we can tell) really matter to the business of living a Christian life; which in turn means it was an evil and damaging dispute: that the dispute itself was far more damaging than the heresy (whichever side was heretical: and of course, both sides may have been in error).
This - if true - would not be at all surprising. If salvation hinged upon our correct understanding and precise expression of advanced philosophical, or linguistic, concepts - then this would run counter to what what we read in the Gospels, and counter to the teachings that the poor and ignorant are most likely to attain salvation (due to their humility and love of God).
Of course, theology is necessary; yet once there is a theology there will be questions, and these questions will seem to require answers, and these answers will satisfy some and lead to further questions among others...
When Christianity focuses on theology, then trouble is in the offing, unless there is already great faith, humility, inner discipline and love of God within which theological exploration may occur.
But even then, the corruptions of the world, the limits of knowledge and the feebleness of reason will tend to lead us astray.
Intellectuals are the worst culprits. Intellectuals love to evaluate others using theological criteria: these are heretical because they believe this - we know they believe it because it is written in their versions of scripture.
Thus intellectuals reason from statements of belief, to the (supposed) implications of belief - which is insecure already; and ignore the actual outcomes in terms of Christian life
Surveying the history of the Christian Church it can be seen that some supposed 'heresies' certainly do not look like they are fatal to salvation; while others (e.g. Arianism, Unitarianism) certainly do look like they are real and fatal heresies.
This distinction between significant and insignificant heresy is not immediately obvious, since a person who adopts what will turn out to be a fatal heresy retains much from their previous state, and their behaviour may not change much immediately - the implications of a heresy will typically become clearly apparent only across the timespan of one or two generations.
For instance, Calvinism in New England was rejected by the heresy of Unitarianism; but Unitarianism only lasted about a generation before it collapsed into Emersonian subjectivist Transcendentalism, which then swiftly (in less than a generation) collapsed into politics (abolition, feminism etc).
What I find dismaying is that people so seldom do what I am doing here: look back to see if what actually happened helps us to understand what is essential, and what inessential, to Christianity; and then to learn from this about how to proceed in future.
Why, for example, has the Eastern Orthodox Church not re-united to undo the separation from Monophysite controversy? Why do not Liberal Christians (if such exist) perceive that the fruits of Liberalism in the Anglican, Roman Catholic, Lutheran (and other) denominations has been the destruction of Christianity?
That these things do not happen shows that the damage from theological schism is usually irreparable - that, in fact, theological schism leads to an even greater, and more damaging, focus on theology: such that after a while nothing else seems to matter.
And the one thing that cannot be perceived is that some apparent theological differences (such as the Monophysite dispute) are not valid grounds for considerations of heresy; and which side was taken made no real difference to real Christians.
It was the taking of sides that was the main problem.