On The Holy Trinity by St. Augustine
Chapter 11.--What is Said Relatively in the Trinity.
12. But whereas, in the same Trinity, some things severally are specially predicated, these are in no way said in reference to themselves in themselves, but either in mutual reference, or in respect to the creature; and, therefore, it is manifest that such things are spoken relatively, not in the way of substance. For the Trinity is called one God, great, good, eternal, omnipotent; and the same God Himself may be called His own deity, His own magnitude, His own goodness, His own eternity, His own omnipotence: but the Trinity cannot in the same way be called the Father, except perhaps metaphorically, in respect to the creature, on account of the adoption of sons. For that which is written, |Hear, O Israel: the Lord our God is one Lord,| ought certainly not to be understood as if the Son were excepted, or the Holy Spirit were excepted; which one Lord our God we rightly call also our Father, as regenerating us by His grace. Neither can the Trinity in any wise be called the Son, but it can be called, in its entirety, the Holy Spirit, according to that which is written, |God is a Spirit;| because both the Father is a spirit and the Son is a spirit, and the Father is holy and the Son is holy. Therefore, since the Father, the Son and the Holy Spirit are one God, and certainly God is holy, and God is a spirit, the Trinity can be called also the Holy Spirit. But yet that Holy Spirit, who is not the Trinity, but is understood as in the Trinity, is spoken of in His proper name of the Holy Spirit relatively, since He is referred both to the Father and to the Son, because the Holy Spirit is the Spirit both of the Father and of the Son. But the relation is not itself apparent in that name, but it is apparent when He is called the gift of God; for He is the gift of the Father and of the Son, because |He proceeds from the Father,| as the Lord says; and because that which the apostle says, |Now, if any man have not the Spirit of Christ, he is none of His,| he says certainly of the Holy Spirit Himself. When we say, therefore, the gift of the giver, and the giver of the gift, we speak in both cases relatively in reciprocal reference. Therefore the Holy Spirit is a certain unutterable communion of the Father and the Son; and on that account, perhaps, He is so called, because the same name is suitable to both the Father and the Son. For He Himself is called specially that which they are called in common; because both the Father is a spirit and the Son a spirit, both the Father is holy and the Son holy. In order, therefore, that the communion of both may be signified from a name which is suitable to both, the Holy Spirit is called the gift of both. And this Trinity is one God, alone, good, great, eternal, omnipotent; itself its own unity, deity, greatness, goodness, eternity, omnipotence.