He wrought first in the heavenly and angelic powers, and such as are first after God and around God. For from no other source flows their perfection and their brightness, and the difficulty or impossibility of moving them to sin, but from the Holy Ghost. And next, in the Patriarchs and Prophets, of whom the former saw Visions of God, or knew Him, and the latter also foreknew the future, having their master part moulded by the Spirit, and being associated with events that were yet future as if present, for such is the power of the Spirit. And next in the Disciples of Christ (for I omit to mention Christ Himself, in Whom He dwelt, not as energizing, but as accompanying His Equal), and that in three ways, as they were able to receive Him, and on three occasions; before Christ was glorified by the Passion, and after He was glorified by the Resurrection; and after His Ascension, or Restoration, or whatever we ought to call it, to Heaven. 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. For it was fitting that as the Son had lived with us in bodily form—so the Spirit too should appear in bodily form; and that after Christ had returned to His own place, He should have come down to us—Coming because He is the Lord; Sent, because He is not a rival God. For such words no less manifest the Unanimity than they mark the separate Individuality.
Gregory of Nazianzus, Oration 41, On Pentecost.
Now the Spirit is not brought into intimate association with the soul by local approximation. How indeed could there be a corporeal approach to the incorporeal? This association results from the withdrawal of the passions which, coming afterwards gradually on the soul from its friendship to the flesh, have alienated it from its close relationship with God. Only then after a man is purified from the shame whose stain he took through his wickedness, and has come back again to his natural beauty, and as it were cleaning the Royal Image and restoring its ancient form, only thus is it possible for him to draw near to the Paraclete. And He, like the sun, will by the aid of thy purified eye show thee in Himself the image of the invisible, and in the blessed spectacle of the image thou shalt behold the unspeakable beauty of the archetype. Through His aid hearts are lifted up, the weak are held by the hand, and they who are advancing are brought to perfection. Shining upon those that are cleansed from every spot, He makes them spiritual by fellowship with Himself. Just as when a sunbeam falls on bright and transparent bodies, they themselves become brilliant too, and shed forth a fresh brightness from themselves, so souls wherein the Spirit dwells, illuminated by the Spirit, themselves become spiritual, and send forth their grace to others. Hence comes foreknowledge of the future, understanding of mysteries, apprehension of what is hidden, distribution of good gifts, the heavenly citizenship, a place in the chorus of angels, joy without end, abiding in God, the being made like to God, and, highest of all, the being made God.
Basil the Great, On the Holy Spirit, IX, 23.