We shall follow the example of Abdemelech, if we believe that the Son and Holy Spirit know all things. This knowledge is attributed in Scripture to the Spirit, and also to the Son. The Son is glorified by the Spirit, as also the Spirit by the Son. Also, inasmuch as we read that the Father, the Son, and the Spirit say and reveal the same things, we must acknowledge in Them a oneness of nature and knowledge. Lastly, that the Spirit searches the deep things of God is not a mark of ignorance, since the Father and the Son are likewise said to search, and Paul, although chosen by Christ, yet was taught by the Spirit.
114. And you, too, shall be Abdemelech, that is, chosen by the Lord, if you raise the Word of God from the depth of Gentile ignorance; if you believe that the Son of God is not deceived, that nothing escapes His knowledge, that He is not ignorant of what is going to be. And the Holy Spirit also is not deceived, of Whom the Lord says: “But when He, the Spirit of Truth, shall come, He shall lead you into all truth.” He Who says all passes by nothing, neither the day nor the hour, neither things past nor things to come.
115. And that you may know that He both knows all things, and foretells things to come, and that His knowledge is one with that of the Father and the Son, hear what the Truth of God says concerning Him: “For He shall not speak from Himself, but what things He shall hear shall He speak, and He shall declare unto you the things that are to come.”
116. Therefore, that you may observe that He knows all things, when the Son said: “But of that day and hour knows no one, not even the Angels of heaven,” He excepted the Holy Spirit. But if the Holy Spirit is excepted from ignorance, how is the Son of God not excepted?
117. But you say that He numbered the Son of God also with the Angels. He numbered the Son indeed, but He did not number the Spirit also. Confess, then, either that the Holy Spirit is greater than the Son of God, so as to speak now not only as an Arian, but even as a Photinian, or acknowledge to what you ought to refer it that He said that the Son knew not. For as man He could [in His human nature] be numbered with creatures Who were created.
118. But if you are willing to learn that the Son of God knows all things, and has foreknowledge of all, see that those very things which you think to be unknown to the Son, the Holy Spirit received from the Son. He received them, however, through Unity of Substance, as the Son received from the Father. “He,” says He, “shall glorify Me, for He shall receive of Mine and shall declare it unto you. All things whatsoever the Father has are Mine, therefore said I, He shall receive of Mine, and shall declare it unto you.” What, then, is more clear than this Unity? What things the Father has pertain to the Son; what things the Son has the Holy Spirit also has received.
119. Yet learn that the Son knows the day of judgment. We read in Zechariah: “And the Lord my God shall come, and all the saints with Him. In that day there shall not be light, but cold and frost, and it shall be one day, and that day is known unto the Lord.” This day, then, was known unto the Lord, Who shall come with His saints, to enlighten us by His second Advent.
120. But let us continue the point which we have commenced concerning the Spirit. For in the passage we have brought forward you find that the Son says of the Spirit: “He shall glorify Me.” So, then, the Spirit glorifies the Son, as the Father also glorifies Him, but the Son of God also glorifies the Spirit, as we said above. He, then, is not weak who is the cause of the mutual glory through the Unity of the Eternal Light, nor is He inferior to the Spirit, of Whom this is true that He is glorified by the Spirit.
122. And you too shall be chosen, if you believe that the Spirit spoke that which the Father spoke, and which the Son spoke. Paul, in fine, was therefore chosen because he so believed and so taught, since, as it is written, God “has revealed to us by His Spirit that which eye has not seen, nor ear heard, nor has entered into the heart of man, the things which God has prepared for them that love Him.” And therefore is He called the Spirit of revelation, as you read: “For God gives to those who thus prepare themselves the Spirit of wisdom and revelation, that He may be known.”
123. There is, then, a Unity of knowledge, since, as the Father, Who gives the Spirit of revelation, reveals, so also the Son reveals, for it is written: “No one knows the Son save the Father, neither does any one know the Father save the Son, and he to whom the Son shall will to reveal Him.” He said more concerning the Son, not because He has more than the Father, but lest He should be supposed to have less. And not unfittingly is the Father thus revealed by the Son, for the Son knows the Father even as the Father knows the Son.
124. Learn now that the Spirit too knows God the Father, for it is written that, “As no one knows the things of a man, save the spirit which is in him, so too the things of God no one knows save the Spirit of God.” “No one,” he says, “knows save the Spirit of God.” Is, then, the Son of God excluded? Certainly not, since neither is the Spirit excluded, when it is said: “And none knows the Father, save the Son.”
125. Therefore the Father, Son, and Holy Spirit are of one nature and of one knowledge. And the Spirit is not to be numbered with all things which were made by the Son, since He knew the Father, Whom (as it is written) who can know save the Son? But the Holy Spirit knows also. What then? When the totality of created things is spoken of, it follows that the Holy Spirit is not included.
126. Now I should like them to answer what it is in man which knows the things of a man. Certainly that must be reasonable which surpasses the other powers of the soul, and by which the highest nature of man is estimated. What, then, is the Spirit, Who knows the deep things of God, and through Whom Almighty God is revealed? Is He inferior in the fullness of the Godhead Who is proved even by this instance to be of one substance with the Father? Or is He ignorant of anything Who knows the counsels of God, and His mysteries which have been hidden from the beginning? What is there that He knows not Who knows all things that are of God? For “the Spirit searches even the deep things of God.”
127. But lest you should think that He searches things unknown, and so searches that He may learn that which He knows not, it is stated first that God revealed them to us through His Spirit, and at the same time in order that you may learn that the Spirit knows the things which are revealed to us through the Spirit Himself, it is said subsequently: “For who among men knows the things of a man, save the spirit of the man which is in him? So, too, the things of God knows no one save the Spirit of God.” If, then, the spirit of a man knows the things of a man, and knows them before it searches, can there be anything of God which the Spirit of God knows not? Of Whom the Apostle said not without a purpose, “The things of God knows no one, save the Spirit of God;” not that He knows by searching, but knows by nature; not that the knowledge of divine things is an accident in Him, but is His natural knowledge.
Source: On the Holy Spirit (New Advent)