When I see St. Gregory the Theologian talking about the Nativity and Theophany being celebrated at the same time, I am sorely tempted to talk about when and where Christmas began to be celebrated as its own unique feast, the fact that Epiphany has a longer provenance in the East than does Christmas, and that all of this is quite interesting given that evidence suggests that Constantinople started to celebrate the Nativity on its own about the time St. Gregory began preaching there.
For those who are interested in a discussion of when Christmas became a Christian feast (particularly in context of the Christianization of the pagan winter solstice) I would direct you here; however, given that we have just celebrated The Entrance of the Theotokos into the Temple — a Great Feast of the Orthodox Church just as the Nativity is — I am more interested in the main point St. Gregory makes in this third section of Oration 38: the Nativity of Christ is a Theophany.
One point about Christmas that I think cannot be emphasized enough is that it is a revelation of God. He demonstrates to us His love and the lengths to which He will go in order to intimately identify Himself with His creation. God becomes man. Christ Himself is a Theophany.
Thus, it is possible to describe all of the 12 Great Feasts of the Orthodox Church as Theophanies — God reveals Himself to us. When one considers feasts like Epiphany, Pentecost and Transfiguration, this idea of Theophany is quite easy to see because God Himself is visibly active in each case. This premise, however, is far more difficult to see with the Entrance of the Theotokos into the Temple.
Firstly, this event is not recorded in Scripture — the record of God’s revelation of Himself to us. Secondly, this celebration focuses on the Virgin Mary, not Christ. The Hymnody of the Church (this from the Doxastikon from Great Vespers of The Entrance), however, helps us to understand how The Entrance is, in fact, a Theophany:
After you were born, O Bride of God and Sovereign Lady, you came to the temple of the Lord to be brought up in the Holy of Holies, as one holy; and Gabriel was then sent to you, the all-blameless, bringing nourishment to you. All the Heavens were astonished, beholding the Holy Spirit make His dwelling in you. Wherefore, O spotless and undefiled Virgin, who are glorified in Heaven and on earth, O Mother of God, save our race.
The Church explicitly states that the Theotokos, as a young little girl, entered into the Holy of Holies. The three OT readings from Great Vespers (Exodus 40:1-5, 9-10, 16, 34-35; Third Book of Book of Kings [1Kings in the Masoretic] 8:1, 3-4, 5 ,6-7, 9, 10-11 and Ezekial 43:27-44:4) make it very clear that such a feat is impossible. In both the Tabernacle built by Moses and the Temple built by Solomon, a cloud descended upon the Holy of Holies so that no one could enter.
This image is reinforced both by Ezekial who states, “Behold, the glory of the Lord filled the house of the Lord; and I fell on my face” and Paul in the Epistle reading from Hebrews who explains that “only the high priest goes, and he but once a year, and not without taking blood which he offers for himself and for the errors of the people” (Hebrews 9:7).
Thus, the Holy of Holies was the place where God’s glory resided, making it impossible to enter save for the High Priest and only once a year with a blood sacrifice. Yet, the Theotokos as young girl is able to freely enter. The answer as to why is from the same hymn above: All the Heavens were astonished, beholding the Holy Spirit make His dwelling in you.
This is a Theophany. God demonstrates His intention for all of humanity: that the Holy Spirit dwell in each and everyone of us so that we, too, can enter into the full glory of God. This reality, of course, is made possible through the life and cooperation of the Theotokos: Let it be done unto me according to your word.
Thus, through our own efforts of preparation (such as the Nativity Fast) and our own cooperation with God we are capable to behold the full glory of God and of entering into the Holy of Holies just as our Lady the Theotokos and Ever-Virgin Mary did as a young child.