St. Ambros’ treatise On the Holy Spirit is written in the form of a letter to the Emperor Gratian, who reigned from A.D. 375 to 383 and, through the influence of Ambrose, was a champion of the Nicene faith over and against both Arianism and its off-shoots as well as paganism. Written in A.D. 381, it coincides with the Second Ecumenical Council which affirmed that the Holy Spirit is God “even as the Father and Son are God: who proceeds from the Father, who with the Father and the Son together is worshipped and together glorified.” These words echo those of the Nicene-Constantinopolitan Creed that Orthodox Christians recite at every Divine Liturgy — the section on the Holy Spirit being added to the Creed at the Second Ecumenical Council, convened at Constantinople.
Ambrose begins the First Chapter of his letter to Gratian by complimenting the Emperor on his decision to restore the Basilica to the Church. One might be tempted to chalk this compliment up to what we today crudely call “brown-nosing;” however, Ambrose insists that this decision had its source in the grace of the Holy Spirit:
So, then, we have received the grace of your faith and the reward of our own; for we cannot say otherwise than that it was of the grace of the Holy Spirit, that when all were unconscious of it, you suddenly restored the Basilica. This is the gift, I say, this the work of the Holy Spirit, Who indeed was at that time preached by us, but was working in you.
He uses this compliment to launch into a defense of the divinity of the Holy Spirit. Thus, he argues that the inspiration for the restoration of the Basilica to the Church did not come from a mere creature (as Arianism and its off-shoots would claim) but God Himself.
This is particularly important to realize today, on the Feast of the Entrance of the Theotokos into the Temple when we sing this Kontakion:
The whole world is filled today with joy and gladness on the Theotokos’s auspicious and resplendent feast, whereon with great voice it cries out: In truth, she is the heavenly tabernacle.
The tabernacle is the tent that God instructed Moses to make in order to house the ark of the covenant. Both the Temple and Orthodox Churches are modeled after the tabernacle. The narthex is where the Hebrews would bring their sacrifices. The nave (where, today, the Orthodox Christian laity worship) is where the sacrificial altar for the burnt and liquid offerings was — where only the priests were allowed. The altar, behind the iconostasis, is where the Holy of Holies was — where the ark of the covenant was — and could only be entered once a year. The tabernacle was where God resided — specifically where the ark was.
According to the Feast of the Entrance of the Theotokos into the Temple, Ioakim and Anna present their daughter Mary to the Temple when she was three years old as a temple virgin. Therein she enters the Holy of Holies in preparation for her role as the Mother of God.
Historically there were two Temples — the first built by Solomon which was destroyed by the Babylonians, and the second built after the Babylonian Exile. The second Temple — the one Mary entered — did not house the ark.
Thus, in her role as the Mother of God — the womb wherein the Incarnate God resided — she is the new ark. She is, as the Kontakion declares, the heavenly tabernacle. This is why most Orthodox Christian Churches have the icon of the Platytera — the Theotokos as a throne whereupon the Christ-child sits — behind the altar. This would be where that ark of the covenant would be in the architecture of the tabernacle.
This reality — in truth, she is the heavenly tabernacle — happens through the descent of the Holy Spirit, whereupon the Incarnate Christ comes to reside in her womb. This same reality happens during every Divine Liturgy. The Holy Spirit descends upon Orthodox Christians and the Gifts. The Gifts become the very Body and very Blood of our Lord, God and Savior Jesus Christ. Then, Orthodox Christians partake. In that moment, we participate in the reality of the Theotokos-as-heavenly-tabernacle because just like her, the Holy Spirit descends and Christ resides in us.
If the Holy Spirit is not True God, of one essence with the Father and the Son this reality is not possible. If the Holy Spirit, through His descent upon us and the gifts, is our source of communion, His being merely a creature and not God would only grant us access to that which we already have — creation. We would have no access to God. Christ’s Incarnation and Crucifixion would be rendered meaningless.
This is why St. Ambrose and the Fathers of the Second Ecumenical Council insisted that the Holy Spirit is God — so that we, like the Theotokos herself, could be a tabernacle of God Himself. Amen.