Church Fathers: Divine Simplicity

“If then any man conceives God to be compound, as accident is in essence, or to have any external envelopment, and to be encompassed, or as if there is aught about Him which completes the essence, so that when we say ‘God,’ or name ‘Father,’ we do not signify the invisible and incomprehensible essence, but something about it, then let them complain of the Council’s stating that the Son was from the essence of God; but let them reflect, that in thus considering they utter two blasphemies; for they make God corporeal, and they falsely say that the Lord is not Son of the very Father, but of what is about Him. But if God be simple, as He is, it follows that in saying ‘God’ and naming ‘Father,’ we name nothing as if about Him, but signify his essence itself. For though to comprehend what the essence of God is be impossible, yet if we only understand that God is, and if Scripture indicates Him by means of these titles, we, with the intention of indicating Him and none else, call Him God and Father and Lord. When then He says, ‘I am that I am,’ and ‘I am the Lord God,’ or when Scripture says, ‘God,’ we understand nothing else by it but the intimation of His incomprehensible essence Itself, and that He Is, who is spoken of.”
(St. Athanasius, De Decretis)
“For He is simple essence, in which quality is not, nor, as James says, ‘any variableness or shadow of turning’ (James 1:17). Accordingly, if it is shown that it is not from virtue (for in God there is no quality, neither is there in the Son), then He must be proper to God’s essence.”
(St. Athanasius, Ad Afros Epistola Synodica)
“Then Niceta began as follows: “Everything that is, is either simple or compound. That which is simple is without number, division, colour, difference, roughness, smoothness, weight, lightness, quality, quantity, and therefore without end. But that which is compound is either compounded of two, or of three, or even of four elements, or at all events of several; and things which are compounded can also of necessity be divided.” The old man, hearing this, said: “You speak most excellently and learnedly, my son.” Then Niceta went on: “Therefore that which is simple, and which is without any of those things by which that which subsists can be dissolved, is without doubt incomprehensible and infinite, knowing neither beginning nor end, and therefore is one and alone, and subsisting without an author. But that which is compound is subject to number, and diversity, and division,—is necessarily compounded by some author, and is a diversity collected into one species. That which is infinite is therefore, in respect of goodness, a Father; in respect of power, a Creator. Nor can the power of creating cease in the Infinite, nor the goodness be quiescent; but He is impelled by goodness to change existing things, and by power to arrange and strengthen them. Therefore some things, as we have said, are changed, and composed of two or three, some of four, others of more elements. But since our inquiry at present is concerning the method of the world and its substance, which, it is agreed, is compounded of four elements, to which all those ten differences belong which we have mentioned above, let us begin at these lower steps, and come to the higher. For a way is afforded us to intellectual and invisible things from those which we see and handle; as is contained in arithmetical instructions, where, when inquiry is made concerning divine things, we rise from the lower to the higher numbers; but when the method respecting present and visible things is expounded, the order is directed from the higher to the lower numbers. Is it not so?””
(The Recognitions of St. Clement, Book 8, Chapter 9)
“It is to declare, that all these were instances of (His) condescension, not the vision of the Essence itself unveiled. For had they seen the very Nature, they would not have beheld It under different forms, since that is simple, without form, or parts, or bounding lines.”
(St. John Chrysostom, Homily 15 on the Gospel of John)
“‘But the operations are varied, and the substance is simple.’ We say that from His operations we know our God; we do not undertake to approach His substance itself. His operations come down to us, but His substance remains inaccessible.”
(St. Basil, Letter 234)
“But if the Father who begot wisdom is also made wise by it, and to be is not to Him the same as to be wise, then the Son is His quality, not His offspring; and there will no longer be absolute simplicity in the Godhead. But far be it from being so, since in truth in the Godhead is absolutely simple essence, and therefore to be is there the same as to be wise.”
(St. Augustine, On the Trinity, Book VII)
“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 (Wisdom 7:22) 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.”
(St. Augustine, City of God, Book XI, Chapter 10)

Hermias. And how, they say, is the divine simple if, in existence on the one hand and in will on the other, it is conceived of separately? For then it would be composite and as though it existed, in a way, out of parts that had come together into a closer unity.

Cyril. Therefore, since, in your view, the divine is simple and exists above all composition (and this view of yours is correct), his will is nothing other than he himself. And if someone says “will,” he indicates the nature of God the Father.

Hermias. So it would appear.
(St. Cyril of Alexandria, Dialogues on the Trinity, Ad Hermiam, Book V)