After proof that the Spirit is the Giver of revelation equally with the Father and the Son, it is explained how the same Spirit does not speak of Himself; and it is shown that no bodily organs are to be thought of in Him, and that no inferiority is to be supposed from the fact of our reading that He hears, since the same would have to be attributed to the Son, and indeed even to the Father, since He hears the Son. The Spirit then hears and glorifies the Son in the sense that He revealed Him to the prophets and apostles, by which the Unity of operation of the Three Persons is inferred; and, since the Spirit does the same works as the Father, the substance of each is also declared to be the same.
130. It has then been proved that like as God has revealed to us the things which are His, so too the Son, and so too the Spirit, has revealed the things of God. For our knowledge proceeds from one Spirit, through one Son to one Father; and from one Father through one Son to one Holy Spirit is delivered goodness and sanctification and the sovereign right of eternal power. Where, then, there is a manifestation of the Spirit, there is the power of God, nor can there be any distinction where the work is one. And therefore that which the Son says the Father also says, and that which the Father says the Son also says, and that which the Father and the Son say the Holy Spirit also says.
131. Whence also the Son of God said concerning the Holy Spirit: “He shall not speak from Himself,” that is, not without the participation of the Father and Myself. For the Spirit is not divided and separated, but speaks what He hears. He hears, that is to say, by unity of substance and by the property of knowledge. For He receives not hearing by any orifices of the body, nor does the divine voice resound with any carnal measures, nor does He hear what He knows not; since commonly in human matters hearing produces knowledge, and yet not even in men themselves is there always bodily speech or fleshly hearing. For “he that speaks in tongues,” it is said, “speaks not to men but to God, for no one hears, but in the Spirit he speaks mysteries.”
132. Therefore if in men hearing is not always of the body, do you require in God the voices of man's weakness, and certain organs of fleshly hearing, when He is said to hear in order that we may believe that He knows? For we know that which we have heard, and we hear beforehand that we may be able to know; but in God Who knows all things knowledge goes before hearing. So in order to state that the Son is not ignorant of what the Father wills, we say that He has heard; but in God there is no sound nor syllable, such as usually signify the indication of the will; but oneness of will is comprehended in hidden ways in God, but in us is shown by signs.
133. What means, then, “He shall not speak from Himself”? This is, He shall not speak without Me; for He speaks the truth, He breathes wisdom. He speaks not without the Father, for He is the Spirit of God; He hears not from Himself, for all things are of God.
134. The Son received all things from the Father, for He Himself said: “All things have been delivered unto Me from My Father.” All that is the Father's the Son also has, for He says again: “All things which the Father has are Mine.” And those things which He Himself received by Unity of nature, the Spirit by the same Unity of nature received also from Him, as the Lord Jesus Himself declares, when speaking of His Spirit: “Therefore said I, He shall receive of Mine and shall declare it unto you.” Therefore what the Spirit says is the Son's, what the Son has given is the Father's. So neither the Son nor the Spirit speaks anything of Himself. For the Trinity speaks nothing external to Itself.
135. But if you contend that this is an argument for the weakness of the Holy Spirit, and for a kind of likeness to the lowliness of the body, you will also make it an argument to the injury of the Son, because the Son said of Himself: “As I hear I judge,” and “The Son can do nothing else than what He sees the Father doing.” For if that be true, as it is, which the Son said: “All things which the Father has are Mine,” and the Son according to the Godhead is One with the Father, One by natural substance, not according to the Sabellian falsehood; that which is one by the property of substance certainly cannot be separated, and so the Son cannot do anything except what He has heard of the Father, for the Word of God endures forever, nor is the Father ever separated from the operation of the Son; and that which the Son works He knows that the Father wills, and what the Father wills the Son knows how to work.
136. Lastly, that one may not think that there is any difference of work either in time or in order between the Father and the Son, but may believe the oneness of the same operation, He says: “The works which I do He does.” And again, that one may not think that there is any difference in the distinction of the works, but may judge that the will, the working, and the power of the Father and the Son are the same, Wisdom says concerning the Father: “For whatsoever things He does, the Son likewise does the same.” So that the action of neither Person is before or after that of the Other, but the same result of one operation. And for this reason the Son says that He can do nothing of Himself, because His operation cannot be separated from that of the Father. In like manner the operation of the Holy Spirit is not separated. Whence also the things which He speaks, He is said to hear from the Father.
137. What if I demonstrate that the Father also hears the Son, as the Son too hears the Father? For you have it written in the Gospel that the Son says: “Father, I thank You that You heard Me.” How did the Father hear the Son, since in the previous passage concerning Lazarus the Son spoke nothing to the Father? And that we might not think that the Son was heard once by the Father, He added: “And I knew that You hear Me always.” Therefore the hearing is not that of subject obedience, but of eternal Unity.
138. In like manner, then, the Spirit is said to hear from the Father, and to glorify the Son. To glorify, because the Holy Spirit taught us that the Son is the Image of the invisible God, and the brightness of His glory, and the impress of His substance. The Spirit also spoke in the patriarchs and the prophets, and, lastly, the apostles began then to be more perfect after that they had received the Holy Spirit. There is therefore no separation of the divine power and grace, for although “there are diversities of gifts, yet it is the same Spirit; and diversities of ministrations, yet the same Lord; and diversities of operations, yet the same God Who works all in all.” There are diversities of offices, not severances of the Trinity.
139. Lastly, it is the same God Who works all in all, that you may know that there is no diversity of operation between God the Father and the Holy Spirit; since those things which the Spirit works, God the Father also works, “Who works all in all.” For while God the Father works all in all, yet “to one is given through the Spirit the word of wisdom; to another the word of knowledge, according to the same Spirit; to another faith, in the same Spirit; to another the gift of healings, in the one Spirit; to another the working of miracles; to another prophecy; to another discerning of spirits; to another various kinds of tongues; to another the interpretation of sayings; but all these works one and the same Spirit, dividing to each one as He will.”
Source: On the Holy Spirit (New Advent)