Although most christian thinkers today reject the doctrine of Divine Simplicity, the view was nearly universally held among the Patristic and Medeival theologians. I provide here a brief passage from Augustine’s City of God XI. 10 where he (seemingly paranthetically, as it interrupts his discourse on angels) ponders on Simplicity and the Three Persons in God.
There is, accordingly, a good which is alone simple, and therefore alone unchangeable, and this is God. By this Good have all others been created, but not simple, and therefore not unchangeable. “Created,” I say — that is, made, not begotten. 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.
It is for this reason, then, that the nature of the Trinity is called simple, because it has not anything which it can lose, and because it is not one thing and its contents another, as a cup and the liquor, or a body and its color, or the air and the light or heat of it, or a mind and its wisdom. For none of these is what it has: the cup is not liquor, nor the body color, nor the air light and heat, nor the mind wisdom. And hence they can be deprived of what they have, and can be turned or changed into other qualities and states, so that the cup may be emptied of the liquid of which it is full, the body be discolored, the air darken, the mind grow silly…
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. In Holy Scripture, it is true, the Spirit of wisdom is called “manifold” because it contains many things in it; but what it contains it also is, and it being one is all these things. For neither are there many wisdoms, but one, in which are untold and infinite treasures of things intellectual, wherein are all invisible and unchangeable reasons of things visible and changeable which were created by it. For God made nothing unwittingly; not even a human workman can be said to do so. But if He knew all that He made, He made only those things which He had known. Whence flows a very striking but true conclusion, that this world could not be known to us unless it existed, but could not have existed unless it had been known to God.