55 But, for my part, I cannot be content by the service of my faith and voice, to deny that my Lord and my God, Your Only-begotten, Jesus Christ, is a creature; I must also deny that this name of 'creature' belongs to Your Holy Spirit, seeing that He proceeds from You and is sent through Him, so great is my reverence for everything that is Yours. Nor, because I know that You alone are unborn and that the Only-begotten is born of You, will I refuse to say that the Holy Spirit was begotten, or assert that He was ever created.
I fear the blasphemies which would be insinuated against You by such use of this title 'creature,' which I share with the other beings brought into being by You. Your Holy Spirit, as the Apostle says, searches and knows Your deep things, and as Intercessor for me speaks to You 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 You through Your Only-begotten? Nothing, except what belongs to You, penetrates into You; nor can the agency of a power foreign and strange to You measure the depth of Your boundless majesty. To You belongs whatever enters into You; nor is anything strange to You, which dwells in You through its searching power.
Source: On the Trinity (New Advent)