On The Holy Trinity by St. Augustine
Chapter 14.--The Father and the Son the Only Beginning (Principium) of the Holy Spirit.
15. But in their mutual relation to one another in the Trinity itself, if the begetter is a beginning in relation to that which he begets, the Father is a beginning in relation to the Son, because He begets Him; but whether the Father is also a beginning in relation to the Holy Spirit, since it is said, |He proceeds from the Father,| is no small question. Because, if it is so, He will not only be a beginning to that thing which He begets or makes, but also to that which He gives. And here, too, that question comes to light, as it can, which is wont to trouble many, Why the Holy Spirit is not also a son, since He, too, comes forth from the Father, as it is read in the Gospel. For the Spirit came forth, not as born, but as given; and so He is not called a son, because He was neither born, as the Only-begotten, nor made, so that by the grace of God He might be born into adoption, as we are. For that which is born of the Father, is referred to the Father only when called Son, and so the Son is the Son of the Father, and not also our Son; but that which is given is referred both to Him who gave, and to those to whom He gave; and so the Holy Spirit is not only the Spirit of the Father and of the Son who gave Him, but He is also called ours, who have received Him: as |The salvation of the Lord,| who gives salvation, is said also to be our salvation, who have received it. Therefore, the Spirit is both the Spirit of God who gave Him, and ours who have received Him. Not, indeed, that spirit of ours by which we are, because that is the spirit of a man which is in him; but this Spirit is ours in another mode, viz. that in which we also say, |Give us this day our bread.| Although certainly we have received that spirit also, which is called the spirit of a man. |For what hast thou,| he says, |which thou didst not receive?| But that is one thing, which we have received that we might be; another, that which we have received that we might be holy. Whence it is also written of John, that he |came in the spirit and power of Elias;| and by the spirit of Elias is meant the Holy Spirit, whom Elias received. And the same thing is to be understood of Moses, when the Lord says to him, |And I will take of thy spirit, and will put it upon them;| that is, I will give to them of the Holy Spirit, which I have already given to thee. If, therefore, that also which is given has him for a beginning by whom it is given, since it has received from no other source that which proceeds from him; it must be admitted that the Father and the Son are a Beginning of the Holy Spirit, not two Beginnings; but as the Father and Son are one God, and one Creator, and one Lord relatively to the creature, so are they one Beginning relatively to the Holy Spirit. But the Father, the Son, and the Holy Spirit is one Beginning in respect to the creature, as also one Creator and one God.