56 But I cannot describe Him, Whose pleas for me I cannot describe. As in the revelation that Your Only-begotten was born of You 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 Your Holy Spirit is from You and through Him, although I cannot by my intellect comprehend it. For in Your spiritual things I am dull, as Your Only-begotten says, Marvel not that I said unto you, you must be born anew.
The Spirit breathes where it will, and you hear 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</em>. Though I hold a belief in my regeneration, I hold it in ignorance; I possess the reality, though I comprehend it not. For my own consciousness had no part in causing this new birth, which is manifest in its effects. Moreover the Spirit has no limits; He speaks when He will, and what He will, and where He will.
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? Your servant John says, indeed, that all things were made through the Son, Who as God the Word was in the beginning, O God, with You. Again, Paul recounts all things as created in Him, in heaven and on earth, visible and invisible. 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 Your Spirit.
With these men, peculiarly Your elect, I will think in these matters; just as, after their example, I will say nothing beyond my comprehension about Your Only-begotten, but simply declare that He was born, so also after their example I will not trespass beyond that which human intellect can know about Your Holy Spirit, but simply declare that He is Your Spirit. May my lot be no useless strife of words, but the unwavering confession of an unhesitating faith!
Source: On the Trinity (New Advent)