Upvote:5
The Augustinian tradition, going back to the early anti-Pelagian writing On Nature and Grace, distinguishes between the human nature and the defect of original sin. We have a defective nature, therefore, and that is what we pass on to our children, but Christ is still consubstantial with us even though He was always without sin. Sin is not part of human nature any more than cancer is part of human nature.
The Formula of Concord (the final Lutheran Confession, 1580) handles this well in its first article. Here's the part that's most immediately relevant:
44] Now, if there were no distinction between the nature or essence of corrupt man and original sin, it must follow that Christ either did not assume our nature, because He did not assume sin, or that, because He assumed our nature, He also assumed sin; both of which ideas are contrary to the Scriptures. But inasmuch as the Son of God assumed our nature, and not original sin, it is clear from this fact that human nature, even since the Fall, and original sin, are not one [and the same] thing, but must be distinguished.
In the Aphthartodocetist controversy, the East phrased the matter in a typically Eastern way (in terms of corruption), just as Augustine phrased it in a typically Western way (in terms of sin). The East sees corruption (the vitiation of our natural powers) as hereditary and sin as the inevitable result in every child of Adam. The West sees sin itself as the hereditary corruption, and everything else as a symptom of that. You can say that Christ took a vitiated nature and lived a perfect life with it, but you can't say that Christ started out as a sinner and then lived a perfect life anyway.
The Lutherans and Calvinists and the whole Augustinian tradition do, however, affirm that Christ took His humanity from Mary, and not from some reservoir of abstract, unfallen Humanity. He is the Son of Mary and the Son of David, and hence also the Son of Adam on His mother's side, even though He is the Son of God on His Father's side (and hence coordinate with Adam, who is also called the son of God in Luke 3:38, and greater than Adam by as much as His Sonship is greater). As such, He took and made use of all the weakness that is characteristic of human nature as we know it, except that He did not yield to temptation. "Like us in every way, save without sin."
In your second-to-last paragraph you confuse Christ's glorification with the revelation of the perfect humanity He had always possessed. In Lutheran theology, at least, His glorification is the revelation of the divine prerogatives that He had always possessed, but had not made use of.