Specifically, Gregory describes humanity as a single reality. He doesn't begin with many individual human beings who can be collected together into a species or set. Rather, Gregory describes one human nature that unfolds across many individual persons over time.
For example, Gregory says that "all humanity is included in the first creation." The creation of Adam is more than the creation of a single individual. Rather, Gregory says "the entire plenitude of humanity was included by the God of all, by His power and foreknowledge, as it were one body." Notice that humanity is "one body" and not a collection of individuals. Humanity is a ontological unity.
True, over time individual persons come into existence separately from each other. But for Gregory, God's creative act encompasses the whole of humanity from beginning to end. Gregory writes that "the man that was manifested at the first creation of the world, and he that shall be after the consummation of all, are alike: they equally bear in themselves the Divine image." The first human being and the last human being participate equally in the one humanity created in God's image.
Gregory pushes that idea further:
"Our whole nature, then, extending from the first to the last, is, so to say, one image of Him Who is..."
The image of God is not located simply in Adam, nor in each individual person separately. Rather, the whole of human nature, stretching from the first human person to the last, is together the one image of God.
Gregory returns to this theme repeatedly. God, he says, "fashioned Man as a whole by His own will to the Divine image," looking upon "the nature of man in its entirety and fulness." God did not create humanity piecemeal, waiting for history gradually to produce the full human race. Rather, in His foreknowledge, God created the entirety of human nature at once.
Gregory explains Genesis 1 in precisely these terms:
"God says, 'Let us make man in our image, after our likeness, and God created man, in the image of God created He him.' Accordingly, the Image of God, which we behold in universal humanity, had its consummation then; but Adam as yet was not... Man, then, was made in the image of God; that is, the universal nature... not part of the whole, but all the fulness of the nature together was so made by omnipotent wisdom."
Gregory distinguishes between "man" in Genesis 1, that is to say "humanity," and the historical person of Adam. The image of God belongs first to "universal humanity," to the whole human nature known by God from beginning to end, of which Adam is the first expression.
History, then, is the gradual manifestation of the humanity that had been created as a whole at the very beginning. This implies, for Gregory, that human history has a definite endpoint because humanity has a definite fullness. Gregory writes:
"For when... the full complement of human nature has reached the limit of the pre-determined measure... the change in existing things will take place in an instant of time."
Human history concludes when the complete human nature, the unified whole, which God intended at the first, finally comes into existence. Once the full number of human persons has been reached, the present age gives way to the age to come.
A related perspective here is how Gregory's understanding of human nature informed how he and Cappadocian Fathers spoke about the Trinity. Or, perhaps, how their view of the Trinity came to inform their view of human nature. Specifically, the Cappadocian Fathers tended to use the words physis (nature) and ousia (essence) interchangeably. The Trinity, for the Cappadocian Fathers, was one physis (nature) and ousia (essence) while existing eternally as three distinct persons (hypostaseis). Seeing a parallel here, Gregory speaks of one human nature (physis) existing in a multitude of human persons (hypostaseis). To be sure, this analogy has its limits, and the Cappadocian language would have to be clarified by later church councils. But for our purposes, all we need note is how Gregory viewed a structural analogy between the Trinity and humanity: one nature and many persons. Human beings are not merely isolated members of the same biological species. Humanity participates in one shared human nature stretching from Adam to the final human person.
With that view now in front of us we can return to the patristic claim that "what is not assumed is not healed." When Christ assumes human nature, he does not exist as an isolated human individual. Rather, Christ's assumes the unified and single whole of human nature in which all human persons, first to last, participate.
