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,” 583 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;” 584 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; 585 for He is the gift of the Father and of the Son, because “He proceeds from the Father,” 586 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,” 587 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. 588 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.
[The reason which Augustin here assigns, why the name Holy Spirit is given to the third person—namely, because spirituality is a characteristic of both the Father and Son, from both of whom he proceeds—is not that assigned in the more developed trinitarianism. The explanation in this latter is, that the third person is denominated the Spirit because of the peculiar manner in which the divine essence is communicated to him—namely, by spiration or out-breathing: spiritus quia spiratus. This is supported by the etymological signification of πνεῦμα, which is breath; and by the symbolical action of Christ in John 20.22John xx. 22, which suggests the eternal spiration, or out-breathing of the third person. The third trinitarian person is no more spiritual, in the sense of immaterial, than the first and second persons, and if the term “Spirit” is to be taken in this the ordinary signification, the “trinitarian relation,” or personal peculiarity, as Augustin remarks, “is not itself apparent in this name;” because it would mention nothing distinctive of the third person, and not belonging to the first and second. But taken technically to denote the spiration or out-breathing by the Father and Son, the trinitarian peculiarity is apparent in the name.
And the epithet “Holy” is similarly explained. The third person is the Holy Spirit, not because he is any more holy than the first and second, but because he is the source and author of holiness in all created spirits. This is eminently and officially his work. In this way also, the epithet “Holy”—which in its ordinary use would specify nothing peculiar to the third person,—mentions a characteristic that differentiates him from the Father and Son.—W.G.T.S.]