Commentary on Holy Scripture

Pentecost

Acts 2

¹ Now when the day of Pentecost had fully come, they were all with one accord in one place.  ² And suddenly there came a sound from heaven, as of a rushing, mighty wind, and it filled the whole house where they were sitting.  ³ Then there appeared to them divided tongues, as of fire, and one sat upon each of them.  ⁴ And they were all filled with the Holy Spirit...

 Commentary from St. Gregory of Nazianzus.  Fifth Theological Oration, 26,26,27,28,29. B#7, pp.210-211.

So that by gradual increase and progress from glory to glory, the light of the Trinity might shine upon the more illuminated ... for this reason it was, I think, that He gradually came to dwell in His disciples. 

He measured Himself out to them according to their capacity to receive Him: at the beginning of the gospel, after the Passion, after the Ascension, making perfect their powers, being breathed upon them, and appearing in fiery tongues....  You see lights breaking upon us gradually, and knowledge of such order of theology, as is better for us to maintain, neither proclaiming things too suddenly nor yet keeping them hidden to the end....  He said that all things should be taught us by the Spirit Himself, made clear at a later time, when such knowledge would be seasonable and capable of being received after our Saviour's restoration; when it would no longer be received with incredulity because of its marvelous character.  For what greater thing than this did either He promise, or the Spirit teach....  If He is not to be worshipped, how can He deify me by baptism?... And indeed from the Spirit comes our new birth, and from the new birth the new creation, and from the new creation our deeper knowledge of the dignity of Him from whom it is derived.... Look at these facts: Christ is born; The Spirit is His forerunner.  He leads Him up.  He works miracles; the Spirit accompanies them.  He ascends; the Spirit takes His place.

St. Gregory of Nazianzus