Who is a Christian? What is a real 'heresy', and which heresies matter and which (ultimately) do not?
It does not seem as if general abstract answers to this question are satisfactory when applied to actual specific individuals. All too often the individual who is 'on paper' a solid Christian, actually seems not to be; and another individual with all kinds of apparently heretical ideas seems to be a real Christian.
As nearly-always, it is a matter of motivation - so the question boils down to a matter of evaluating another person's motivation.
Of course, such evaluation is prone to error, since we lack direct mind-to-mind access to the motivations of another; but on the other hand, Life depends on being able to judge motivations. Actual life is impossible without such an ability; so we can, do and must judge motivations of other people.
What, then, should we be looking for in evaluating whether or not someone is a Christian?
I think this question to ask is along-the-lines-of: Would he follow Jesus through death to eternal resurrected life?
So it is not the conceptualisation of Jesus that is crucial, but the attitude to Jesus.
When it is said that 'Jesus is both God and Man' - this ought not to be regarded at the level of philosophical explanation; but rather a way of saying that Jesus needs to be (in our hearts) both personal and divine for us to want to follow him (as a person) and for him to be able to give us life eternal (as divine).
Someone who regarded Jesus in their hearts as wholly a person (not divine) - say as a great teacher or prophet - may love Jesus; but such a Jesus would not be able to offer resurrection, so could not be followed through death.
At the other extreme, someone who regarded Jesus as wholly divine but not a person, somebody who saw Jesus as essentially a powerful abstraction - such as a creative force, faster spiritual vibration, high-frequency influence on matter, or a glowing light of joy - would not be able to love or follow such an impersonal entity.
I hope this gives the idea of what I am talking about. But what makes a person a Christian or not is not captured by such definitions - the definitions are secondary. The primary reality is the motivation of the heart with respect to Jesus.
Analysis of what a person knows or says about the nature of Jesus is not necessarily relevant, any more than we can evaluate the real motivations of a person by recording and analysing their speech - because people lie, people err or lack intellectual capacity, people deceive themselves as to the truth of their own motivations...
Because we need to know whether other people rally are Christians, there is no way of evading the need for a direct, intuitive, personal evaluation of the motivation of the other - and we need to have faith that we can and are able to make such evaluations; that God would not have left us bereft of such a vital ability.