On The Holy Trinity by St. Augustine
Chapter 5.--The Holy Spirit Also is Equal to the Father and the Son in All Things.
7. Wherefore also the Holy Spirit consists in the same unity of substance, and in the same equality. For whether He is the unity of both, or the holiness, or the love, or therefore the unity because the love, and therefore the love because the holiness, it is manifest that He is not one of the two, through whom the two are joined, through whom the Begotten is loved by the Begetter, and loves Him that begat Him, and through whom, not by participation, but by their own essence, neither by the gift of any superior, but by their own, they are |keeping the unity of the Spirit in the bond of peace;| which we are commanded to imitate by grace, both towards God and towards ourselves. |On which two commandments hang all the law and the prophets.| So those three are God, one, alone, great, wise, holy, blessed. But we are blessed from Him, and through Him, and in Him; because we ourselves are one by His gift, and one spirit with Him, because our soul cleaves to Him so as to follow Him. And it is good for us to cleave to God, since He will destroy every man who is estranged from Him. Therefore the Holy Spirit, whatever it is, is something common both to the Father and Son. But that communion itself is consubstantial and co-eternal; and if it may fitly be called friendship, let it be so called; but it is more aptly called love. And this is also a substance, since God is a substance, and |God is love,| as it is written. But as He is a substance together with the Father and the Son, so that substance is together with them great, and together with them good, and together with them holy, and whatsoever else is said in reference to substance; since it is not one thing to God to be, and another to be great or to be good, and the rest, as we have shown above. For if love is less great therein [i.e. in God] than wisdom, then wisdom is loved in less degree than according to what it is; love is therefore equal, in order that wisdom may be loved according to its being; but wisdom is equal with the Father, as we have proved above; therefore also the Holy Spirit is equal; and if equal, equal in all things, on account of the absolute simplicity which is in that substance. And therefore they are not more than three: One who loves Him who is from Himself, and One who loves Him from whom He is, and Love itself. And if this last is nothing, how is |God love|? If it is not substance, how is God substance?