On The Holy Trinity by St. Augustine
Chapter 3.--Some Things Concerning the Holy Spirit are to Be Understood According to the One Rule Only.
5. For even of the Holy Spirit, of whom it is not said, |He emptied Himself, and took upon Him the form of a servant;| yet the Lord Himself says, |Howbeit, when He the Spirit of Truth is come, He will guide you into all truth. For He shall not speak of Himself, but whatsoever He shall hear that shall He speak; and He will show you things to come. He shall glorify me; for He shall receive of mine, and shall show it unto you.| And except He had immediately gone on to say after this, |All things that the Father hath are mine; therefore said I, that He shall take of mine, and shall show it unto you;| it might, perhaps, have been believed that the Holy Spirit was so born of Christ, as Christ is of the Father. Since He had said of Himself, |My doctrine is not mine, but His that sent me;| but of the Holy Spirit, |For He shall not speak of Himself, but whatsoever he shall hear, that shall He speak;| and, |For He shall receive of mine, and shall show it unto you.| But because He has rendered the reason why He said, |He shall receive of mine| (for He says, |All things that the Father hath are mine; therefore said I, that He shall take of mine|); it remains that the Holy Spirit be understood to have of that which is the Father's, as the Son also hath. And how can this be, unless according to that which we have said above, |But when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth which proceedeth from the Father, He shall testify of me|? He is said, therefore, not to speak of Himself, in that He proceedeth from the Father; and as it does not follow that the Son is less because He said, |The Son can do nothing of Himself, but what He seeth the Father do| (for He has not said this according to the form of a servant, but according to the form of God, as we have already shown, and these words do not set Him forth as less than, but as of the Father), so it is not brought to pass that the Holy Spirit is less, because it is said of Him, |For He shall not speak of Himself, but whatsoever He shall hear, that shall He speak;| for the words belong to Him as proceeding from the Father. But whereas both the Son is of the Father, and the Holy Spirit proceeds from the Father, why both are not called sons, and both not said to be begotten, but the former is called the one only-begotten Son, and the latter, viz. the Holy Spirit, neither son nor begotten, because if begotten, then certainly a son, we will discuss in another place, if God shall grant, and so far as He shall grant.