St.-Gregory-NazianzenHe [the Holy Spirit] worked…in the disciples of Christ…on three occasions—before Christ was glorified by the Passion; and after He was glorified by the Resurrection; and after His Ascension….

Now the first of these manifests Him—the healing of the sick and casting out of evil spirits, which could not be apart from the Spirit;

and so does that breathing upon them after the Resurrection, which was clearly a divine inspiration;

and so too the present distribution of the fiery tongues, which we are now commemorating.

But the first manifested Him indistinctly, the second more expressly, this present one more perfectly, since He is no longer present only in energy, but as we may say, substantially, associating with us, and dwelling in us.

[…] And therefore He came after Christ, that a Comforter should not be lacking unto us; but “another Comforter”, that you might acknowledge His co-equality.

For this word “another” marks an “alter ego”, a name of equal Lordship, not of inequality.  For “another” is not said, I know, of different kinds, but of things consubstantial.

And He came in the form of tongues because of His close relation to the Word.  And they were of fire, perhaps because of His purifying power…, or else because of His Substance.  For our God is a consuming fire….

And the tongues were cloven, because of the diversity of gifts. And they sat to signify His royalty and rest among the saints, and because the cherubim are the throne of God.

And it took place in an upper chamber …, because those who should receive it were to ascend and be raised above the earth; for also certain upper chambers are covered with divine waters, by which the praise of God are sung.

And Jesus Himself in an upper chamber gave the communion of the Sacrament to those who were being initiated into the higher mysteries, that thereby might be shown on the one hand that God must come down to us, as I know He did of old to Moses;

and on the other that we must go up to Him, and that so there should come to pass a communion of God with men, by a coalescing of the dignity.

For as long as either remains on its own footing, the one in His glory the other in his lowliness, so long the goodness of God cannot mingle with us, and His loving-kindness is incommunicable, and there is a great gulf between, which cannot be crossed;

and which separates not only the rich man from Lazarus and Abraham’s Bosom which he longs for, but also the created and changing natures from that which is eternal and immutable.

Gregory Nazianzen (c.330-390): Oration 41 (on Pentecost), 11-12.