those who do hold to incarnational sonship I do not believe are heretical, nor are they fundamentally denying the ontological relationship of the son to the father.
Fred,
Your post is smoke. I’m sorry. Let’s define our terms. Those who hold to the “incarnational sonship”, as did John, deny the eternal sonship. Accordingly, they deny the eternal-ontological Son-Father relationship.
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In John's mind, at the time of teaching through Hebrews, his understanding was that it had incarnational connotations. Thus, the title of the "begotten son" was an incarnational title, not one that the Second person of the Godhead held BEFORE the decree to redeem and Christ was appointed to step into humanity.
Unless titles are meaningless, when John affirmed that “begotten” referred strictly to the incarnation, he by necessary consequence denied the eternal-begotten Son-Father relationship which is a denial of the ontological Trinity since the Persons of the Trinity are who they are by the relationship they hold to the other persons of the Trinity.
Did John deny the eternal sonship of the Second Person? Yes, by saying that the Son was begotten in time and became the Son.
Is the eternal Sonship essential to the Son and to the Trinity? Yes
Therefore, John denied an essential property of the eternal Son and the eternal ontological, Trinity.
Was this heresy? Of course it was. If you wish to deny John's former heresy, please interact with the specifics of the case. It's not germane that John believed in the eternal Second Person or that he affirmed Jesus' sonship. Those points are not in dispute.