Augustine of Hippo remarks:16 "It is said of the Holy Spirit, ‘He keeps on proceeding from the Father’ [John 15:26]…. The Father begot; the Son was begotten…. Also the Holy Spirit is One with them — since these Three are one [First John 5:6-8]…. The gift of God in respect of the Holy Spirit, means to proceed from the Father…. Neither can we say that the Holy Spirit does not also proceed from the Son. For the same Spirit is not without reason said to be the Spirit both of the Father and of the Son….
"The Holy Spirit…manifested His substance, co-eternal with the Father and the Son — and alike with Them, unchangeable… I would boldly say that the Father, Son, and Holy Spirit [are] of one and the same substance — God the Creator, the Omnipotent Trinity — [and] work indivisibly…. The Holy Spirit is the Spirit both of the Father and of the Son…. He is the gift of the Father and of the Son — because ‘He proceeds from the Father’ [and from the Son in John 14:26 & 15:26]….