Tell me, then, whoever you are who deny the Godhead of the Holy Spirit. The Spirit could not be liable to sin, Who rather forgives sin. Does an Angel forgive? Does an Archangel? Certainly not, but the Father alone, the Son alone, and the Holy Spirit alone. Now no one is unable to avoid that which he has power to forgive.
But perhaps some one will say that the Seraph said to Isaiah: “Behold, this hath touched thy lips, and shall take away thine iniquities, and purge away thy sins” [Is. 6:7]. Shall take away, he says, and shall purge, not I will take away, but that fire from the altar of God, that is, the grace of the Spirit. For what else can we piously understand to be on the altar of God but the grace of the Spirit? Certainly not the wood of the forests, nor the soot and coals. Or what is so in accordance with piety as to understand according to the mystery that it was revealed by the mouth of Isaiah that all men should be cleansed by the passion of Christ, Who as a coal according to the flesh burnt up our sins, as you read in Zechariah: “Is not this a brand cast forth from the fire? And that was Joshua clothed in filthy garments” [Zech. 3:2,3].
Lastly, that we may know that this mystery of the common redemption was most clearly revealed by the prophets, you have also in this place: “Lo, it hath taken away thy sins” [Zech. 3:4]; not that Christ put aside His sins Who did no sin, but that in the flesh of Christ the whole human race should be loosed from their sins.
But even if the Seraph had taken away sin, it would have been as one of the ministers of God appointed to this mystery. For thus said Isaiah: “For one of the Seraphim was sent to me” [Is. 6:6].
The Spirit, also, is indeed said to be sent, but the Seraph to one, the Spirit to all. The Seraph is sent to minister, the Spirit works a mystery. The Seraph performs what is commanded, the Spirit divides as He wills. The Seraph passes from place to place, for he does not fill all things, but is himself filled by the Spirit. The Seraph comes down with a certain mode of passage according to his nature, but we cannot think this of the Spirit, of Whom the Son of God says: “When the Paraclete shall come, even the Spirit of Truth, Whom I send unto you, Who proceedeth from the Father” [John 15:26].
For if the Spirit proceeds from a place and passes to a place, both the Father Himself will be found in a place, and the Son likewise. If He goes forth from a place, Whom the Father or the Son sends, certainly the Spirit passing from a place, and making progress, seems to leave, according to those impious interpretations, both the Father and the Son like some material body.
I am saying this with reference to those who say that the Spirit comes down by movement. But neither the Father, Who is above all not only of corporeal nature, but also of the invisible creation, is circumscribed in any place; nor is the Son, Who, as the Worker of all creation, is above every creature, enclosed by the places or times of His own works; nor is the Spirit of Truth as being the Spirit of God, circumscribed by any corporeal limits, Who since He is incorporeal is far above the whole rational creation through the ineffable fulness of His Godhead, having over all things the power of breathing where He wills, and of inspiring as He wills [John 3:8].
The Spirit is not, then, sent as it were from a place, nor does He proceed as from a place, when He proceeds from the Son, as the Son Himself, when He says, “I came forth from the Father, and am come into the world” [John 16:28], destroys all fancies, which can be reckoned as from place to place. In like manner, also, when we read that God is within or without, we certainly do not either enclose God within anybody or separate Him from anybody, but weighing these things in a deep and ineffable estimation, we comprehend the hiddenness of the divine nature.
Lastly, Wisdom so says that she came forth from the mouth of the Most High [Eccles. 24:5], as not to be external to the Father, but with the Father; for “the Word was with God” [John 1:1]; and not only with God but also in God; for He says: “I am in the Father and the Father is in Me” [John 14:10]. But neither when He goes forth from the Father does He retire from a place, nor is He separated as a body from a body; nor when He is in the Father is He as if a body enclosed as it were in a body. The Holy Spirit also, when He proceeds from the Father and the Son, is not separated from the Father nor separated from the Son. For how could He be separated from the Father Who is the Spirit of His mouth? Which is certainly both a proof of His eternity, and expresses the Unity of this Godhead. (“On the Holy Spirit,” Book I, 112-20; Nicene and Post-Nicene Fathers, Second Series, Vol. X [Grand Rapids, Michigan: Wm. B. Eerdmans publishing Company, 1983 reprint], pp. 108-09)