Jesus being “not of this cosmos [κόσμου]” (John 8.23) as proof of his unique ontological unity definitely does not make sense of the Book of John, because Jesus uses this exact same Greek phrasing for his disciples later in the text, which would bestow ontological unity on them too ...
Yes, by incorporation into Himself. Both John and Paul understood it that way. Scripture speaks of sonship, whereas later theologies speak of theosis.
Your conclusion rests entirely on a later metaphysical way of looking at the world anyway: you are either part of the created cosmos, or you share the uncreated nature of God. That is not how first century Jews thought.
I disagree, and I rather think it's just as the early Jewish-Christians thought.
Bart Ehrman has something to say about Paul's 'High Christology':
The 'Christ Hymn' of Philippians 2:6-10 is, in his words, "
an extremely high Christology." For Paul, Christ is a divine being come into the world; and was made
equal with God:
"Jesus, who, subsisting in a god’s form (ie 'in the manner of'), did not deem existing in the manner of a god a thing to be grasped (ie 'held onto'), but instead emptied himself ('divested, emptied, impoverished)', taking a slave’s form, coming to be in a likeness of human beings; and, being found in appearance as a human being ('outward aspect as opposed to inward reality'), he reduced ('humbled' or 'abased') himself, becoming obedient all the way to death, and a death by a cross. For which reason God also exalted him on high and graced him with the name that is above every name, that at the name of Jesus every knee—of beings heavenly and earthly and subterranean—should bend, and every tongue gladly confess that Jesus the Anointed is Lord, for the glory of God the Father."
(Hart's explanation of his translated in parenthesis)
Galatians 4:4: "When the fullness of time came, God sent his son, born from a woman, born under the law." This statement makes sense if Paul believes that Christ was in fact a pre-existent divine being.
In the Hebrew Scriptures, God’s "Wisdom" can be likened to a divine hypostasis; as somnething of God that takes on its own form and existence. 'Wisdom' as a divine agent through whom God creates all things (cf Proverbs 8) – as God's Wisdom, it is God, and an image of God, or God manifest in a Divine Act.
Ehrman argues that at the least, Paul sees Jesus as "the Angel of the Lord" coming into this world: "the glory of Christ, who is the likeness of God" (2 Corinthians 4:4). But as we see in Exodus 3, an "angel of the Lord" appears to him in the burning bush, but then says "I am the God of thy father" (3:6) – a fluid continuity.
Paul declares:
"For even though there are those who are called gods, whether in heaven or on earth (as indeed there are many gods and many lords), yet for us there is one God, the Father—out of whom come all things, and we for him—and one Lord, Jesus the Anointed—through whom
come all things, and we through him" (1 Corinthians 8:5-6)
Hart details this at some length in a footnote to his translation of the NT:
"Paul should be taken fairly literally in these two verses: He really means that, in a sense, there are such things as “gods” in heaven and earth, though as a pious Jew and Christian he would more naturally call them angels or demons. Most Jews, Christians, and educated pagans of late antiquity drew an absolute distinction between, on the one hand, the spiritual or divine powers that rule the nations and inhabit the cosmos and, on the other, the one God who is the source of existence from whom everything comes forth (gods no less than other limited beings). For Paul, these “powers on high,” “archons,” and so on are the gods worshipped by the several nations, but are ultimately only angelic governors of the cosmos, often either rebellious or incompetent; this seems to include even the angel governing Israel, who, according to Galatians, delivered a defective version of the Law to Moses. In Paul's time, the idea of angelic “gods of the nations” would have been, for instance, an unproblematic interpretation of Deuteronomy 32:8-9, which describes God as dividing the nations among the “sons of God [El],” as well as 32: 43, in which these same sons of God, along with the nations they govern, are called to make obeisance to God (in the Rabbinic Masoretic Text
of the Hebrew, which is a later synthetic redaction, the phrase in v. 8 becomes “sons of Israel,” but in the Septuagint—the favored text of Paul and much of the Greek-speaking Diaspora—it was still “sons of God” or perhaps, in some copies, “angels of God”; and in v. 43 the Masoretic Text omits the reference to the sons of God and the angels of the nations altogether, though, again, they are still present in the Septuagintal version). As will emerge in chapter fifteen below, it is a large part of Paul's understanding of the gospel that these cosmic gods have been conquered and placed in proper order by Christ and will, at the end of time, be handed over in proper subordination to the Father so that God may be “all in all.”
To return to Ehrman:
"It is worth stressing that Paul does indeed speak about Jesus as God, as we have seen. That does not mean that Christ is God the Father Almighty. Paul certainly thought Jesus was God in a certain sense – but he does not think that he was the Father. He was an angelic, divine being before coming into the world; he was the Angel of the Lord; he was eventually exalted to be equal with God and worthy of all of God’s honor and worship. And so I now have no trouble recognizing that in fact Paul could indeed flat out call Jesus God, as he appears to do in Romans 9:5."
"If someone as early in the Christian tradition as Paul can see Christ as an incarnate divine being, it is no surprise that the same view emerges later in the tradition as well. Nowhere does it emerge more clearly or forcefully than in the Gospel of John."
And to echo Hart's note, if we agree that Jesus is not the Father, and we do, and place Him on the scale of Divine Beings, then He is at the very top, alone and unique, with the rest arrayed beneath Him, to be judged at the end of the age. Quite how this relationship works is part of the Mystery: Jesus is God, but He is not the Father, but He is not other than, nor another, God ...