Having left his discussion of theology (which Orthodox Christians today might call a discussion about the essence of God), St. Gregory begins a meditation upon the economy of God (which corresponds to the activity, energy, hypostatic reality and grace of God). While we can never comprehend the essence of God, we can fully participate in this economy of God.
This distinction made by St. Gregory is important because it allows us to at least acknowledge and to try and work within the limitations of the mind and language to talk about the experience of the Church and what God has revealed Himself to be. The reality that God is One and Three at the same time can be appreciated. The oneness of God is communicated by the essence of God and the fact that God is Father, Son and Holy Spirit is communicated by the hypostatic reality (the personhood) of God.
Note that when St. Gregory begins to talk about how God shares His goodness with His creation that he begins to talk about the persons of God: “accomplished by the Word and perfected by the Spirit.” Our ability (and therefore the means through which all of creation is able) to participate in the reality of God occurs in what we call the personhood of God. We experience and have a relationship with the persons of the Trinity: through the descent of the Holy Spirit we are able to partake of the Son and become children of the Father.
In his second universal letter, Peter states that
he has granted to us his precious and very great promises, that through these you may escape from the corruption that is in the world because of passion, and become partakers of the divine nature.
If this partaking of the divine nature happened at an essential level (we partake of the essence of God), then creation would necessarily be eternal. God does not change. Therefore there never was a time when He wasn’t Father. It follows that there was never a time when He wasn’t Creator, either. Thus, creation is eternal.
This logic, however, is rejected by the Church (it is found in the musings of Origen and was declared heresy at the Fifth Ecumenical Council in A.D. 553) because it denies the experience of the Church. Creation has a beginning, and therefore, by nature (essence), has an end. In essence, we are changeable (we become and then cease to be). Even the most holy of all creation — the angels — are changeable (movable). St. Gregory:
I am persuaded to consider and say that they are not immovable but only difficult to move on account of the one who was called Lucifer
The distinction between the ousia (aka essence, nature, “theology”) and hypostases (aka activities, persons, “economy”) of God allow us to break the logic of Origen and express the experience of the Church in language that makes the ineffable communicable.
In other words, while we can never know the essence of God, we can partake of the hypostatic reality of God. This linguistic distinction, while not wholly comprehensible, does allow us to begin to understand how it is possible how the Church has experienced the revelation that God is One in essence and in Three hypostases. We can see how He is both unknowable and knowable, that He is both Father and Creator and that the Nativity of Christ is not only possible, but one of the greatest moments in all of history.