I cannot be content…to deny that my Lord and my God, Thy Only-begotten, Jesus Christ, is a creature.
I must also deny that this name of ‘creature’ belongs to Thy Holy Spirit, seeing that He proceeds from Thee and is sent through Him, so great is my reverence for everything that is Thine.
Nor, because I know that Thou alone art unborn and that the Only-begotten is born of Thee, will I refuse to say that the Holy Spirit was begotten, or assert that He was ever created.
[…] Thy Holy Spirit, as the Apostle says, searches and knows Thy deep things, and as Intercessor for me speaks to Thee words I could not utter.
And shall I express or rather dishonour, by the title ‘creature,’ the power of His nature, which subsists eternally, derived from Thee through Thine Only-begotten?
Nothing, except what belongs to Thee, penetrates into Thee; nor can the agency of a power foreign and strange to Thee measure the depth of Thy boundless majesty.
To Thee belongs whatever enters into Thee; nor is anything strange to Thee, which dwells in Thee through its searching power.
But I cannot describe Him, Whose pleas for me I cannot describe.
As in the revelation that Thy Only-begotten was born of Thee before times eternal, when we cease to struggle with ambiguities of language and difficulties of thought the one certainty of His birth remains.
So I hold fast in my consciousness the truth that Thy Holy Spirit is from Thee and through Him, although I cannot by my intellect comprehend it.
For in Thy spiritual things I am dull, as Thy Only-begotten says…: The Spirit breathes where It will, and thou hearest the voice of It; but dost not know whence it comes or whither It goes. So is every one who is born of water and of the Holy Spirit (John 3:7-8).
[…] Since, then, the cause of His coming and going is unknown, though the watcher is conscious of the fact, shall I count the nature of the Spirit among created things, and limit Him by fixing the time of His origin?
Thy servant John says, indeed, that all things were made through the Son, Who as God the Word was in the beginning, O God, with Thee. Again, Paul recounts all things as created in Him, in heaven and on earth, visible and invisible (Col. 1:16).
And, while he declared that everything was created in Christ and through Christ, he thought, with respect to the Holy Spirit, that the description was sufficient, when he called Him Thy Spirit.
Hilary of Poitiers (c.300-368): On the Trinity, 12, 55-56.