The standard Catholic (Roman or Orthodox) understanding of the Divine Nature is that it is a simplicity. The standard complaint of the Orthodox against the Romans is that they make too much of this simplicity and forget entirely the Energies of God. That is, God is both Essence and Energies. This is highly technical and, as is often the case, confusing. I would like to offer some thoughts on perhaps understanding both positions in terms of each other, and perhaps thereby begin to show a synthesis.
What the Orthodox view seems to leave out, at least in the discussions I have had, is the act of God’s existence. Aquinas distinguishes real being as existence and essence. That is, the activity of existence and the whatness of essence. Real beings are “acting what-nots” to put it quasi-technically. This, it seems to me, is a wonderful insight. It is also, however, the insight that gives Roman Catholics their reputation for extremely strong ideas of Divine Simplicity, to the exclusion of the Divine Energies. That is, given that God is both Essence and Existence, but his essence cannot be a limiting essence (as essence is for all other beings), his essence must be unlimited and thus God’s essence IS existence and nothing less.
Now the Divine Energies, according to the Orthodox, are precisely the activities and actions of God. They are his manifold presence, as it were, the way that God makes himself known. This, however, sounds to my ears nearly to be exactly what the Thomistic understanding of the activity of real being to be. That is, for the Thomist one only comes to know any real being insofar as that being is in activity (in act) and, as this name for a real metaphysical principle may suggest, is not static but rather dynamic. The act of being does just that: it acts!
Now, God’s existence is pure act, pure existence. But pure act is pure activity and, being unlimited, is unlimited activity. It is precisely through this Act of God that we know him, insofar as he allows himself to be known. Notice that even for the Thomist we do not (ordinarily) know a beings essence (even though we may know a handful of propositions concerning what essence is and what it does, etc). That is, I do not know my neighbor’s essence, but I only know them insofar as I see them in their activity (and may thus infer much about them). But I can never, as it were, “get to the thing in itself”. Kant seems to have been correct about that much.
Ok, here’s the suggestion (or the surmising). Can it not be that the Divine Energies are not a distinct and separate feature of God in that God has both Essence AND Energies, and also existence, but rather that God is Essence and Existence, and the Divine Energies simply ARE the energies of God’s existence? That is, they are the ways that God makes manifest his existence, as the outpouring of his essence which is, simply, to exist. So God is simple in his essence, there is no shadow of turning with him, yet is actively active through the energies of God which are, precisely, the manifold ways in which God demonstrates his existence to the world. We do not know God’s essence but we do know God’s energies, his activities.
But, the Orthodox will be quick (and correct) to respond that God’s energies are myriad and changing, now this now that, now demonstrated in this way now in that, now grace, now life, now presence, now knowledge, now divine love, etc. How could the Thomist account for that with merely essence and existence? Are there not also, in addition to that, energies? Well, we forget here that existence is activity, it is active and not passive, it is not inert and sterile but abundant and moving, as it were. My essence limits the ways in which my existence is made manifest but God’s does not. God is fully capable of a multitude and multiplicity of activities (energies). So, perhaps it is possible, using the terminology that we in the East and West already use, to find a way to reconcile at least this dividing issue between us. As an Orthodox I see no reason why we cannot make substantial use of Thomas’ philosophical works (pun intended), making alterations where needed and helping the saint along where he has forgotten the light of the Christian East.
I defer further inquiry of the topic to the expert: David Bradshaw at the University of Kentucky. He has some excellent short articles and a book devoted to these matters.