For that which is begotten of the simple Good is simple as itself, and the same as itself. These two we call the Father and the Son; and both together with the Holy Spirit are one God; and to this Spirit the epithet Holy is in Scripture, as it were, appropriated. And He is another than the Father and the Son, for He is neither the Father nor the Son. I say “another,” not “another thing,” because He is equally with them the simple Good, unchangeable and co-eternal. And this Trinity is one God; and none the less simple because a Trinity. For we do not say that the nature of the good is simple, because the Father alone possesses it, or the Son alone, or the Holy Ghost alone; nor do we say, with the Sabellian heretics, that it is only nominally a Trinity, and has no real distinction of persons; but we say it is simple, because it is what it has, with the exception of the relation of the persons to one another. For, in regard to this relation, it is true that the Father has a Son, and yet is not Himself the Son; and the Son has a Father, and is not Himself the Father. But, as regards Himself, irrespective of relation to the other, each is what He has; thus, He is in Himself living, for He has life, and is Himself the Life which He has.
According to this, then, those things which are essentially and truly divine are called simple, because in them quality and substance are identical, and because they are divine, or wise, or blessed in themselves, and without extraneous supplement.
— City of God. XI.10