Augustine of Hippo, on John 15:26

“But when the Paraclete cometh, whom I will send you from the Father, the Spirit of truth, who proceedeth from the Father, he shall give testimony of me.”

“(Tr. xcix. 6, et sq.) If it be asked here whether the Holy Ghost proceeds from the Son also, we may answer thus: The Son is the Son of the Father alone, and the Father is the Father of the Son only; but the Holy Spirit is not the Spirit of one, but of both; since Christ Himself saith, The Spirit of your Father which speaketh in you. (Matt. 10:20) And the Apostle says, God hath sent the Spirit of His Son into your hearts. (Gal. 4:6) This indeed, I think, is the reason why He is called peculiarly the Spirit. For both of the Father and the Son separately we may pronounce, that each is a Spirit. But what each is separately in a general sense, He who is not either one separately, but the union of both, is spiritually. But if the Holy Spirit is the Spirit of the Son, why should we not believe that He proceeds from the Son? Indeed if He did not proceed from the Son, Christ would not after the resurrection have breathed on His disciples, and said, Receive ye the Holy Ghost. (John 20:29) This too is what is meant by the virtue which went out of Him, and healed all. (Luke 6.) If the Holy Ghost then proceeds both from the Father and the Son, why does Christ say, Who proceedeth from the Father? He says it in accordance with His general way of referring all that He has to Him from whom He is; as where He says, My doctrine is not Mine, but His that sent Me. If the doctrine was His, which He says was not His own, but the Father’s, much more does the Holy Spirit proceed from Him, consistently with His proceeding from the Father. From whom the Son hath His Godhead, from Him He hath it that the Holy Ghost proceedeth from Him. And this explains why the Holy Ghost is not said to be born, but to proceed. For if He were born, He would be the Son of both Father and Son, an absurd supposition; for if two together have a Son, those two must be father and mother. But to imagine any such relation as this between God the Father, and God the Son, is monstrous. Even the human offspring does not proceed from father or mother at the same time; when it proceeds from the father, it does not proceed from the mother. Whereas the Holy Spirit does not proceed from the Father into the Son, and from the Son into the creature to be sanctified; but proceeds from Father and Son at once. And if the Father is life, and the Son is life, so the Holy Ghost is life also. Just then as the Father when He had life in Himself, gave also to the Son to have life in Himself; so He gave to the Son also that life should proceed from Him, even as it proceeded from Himself.”

Imported from an open dataset — not yet checked against the printed edition.