Far from being a ‘supreme being’, a nameless deity beyond the world who is ultimately in charge of everything, God is maxime ens, who enjoys being in the highest possible degree. Such a reality must be thought to be utterly simple, but simple in the ‘concrete’ sense of perfection, including, in its simple being, the perfection of all things. And, as implied by his universal perfection, God is self-diffusive goodness, the abundant source of all the good gifts which creatures receive from him, among which the gift of ‘being’ occupies the first place. Consequent upon its simplicity, the reality of God must be thought to be infinite, but infinite in the sense of being most intimately present in each thing, causing it to be from within. And also, as implied by its simplicity, it must be thought to be absolutely unchangeable, but in the sense of being enduringly present to everything which changes over time. The mark of subsistence as qualifying the simplicity of the divine being appears here to be of crucial importance. It is not the subsistence of a supreme substance, conceived somehow as inert and static, enclosed in itself, prior to its creative activity with respect to the world of creatures; rather, the divine essence is the full and unrestricted actuality (actus purus) of being, which, by nature, tends to communicate its actuality to other things by letting them share in being.
There is something in Thomas’ conception of God as ipsum esse per se subsistens that does not fit very well into the picture of ‘classical theism’. Classical theism, as it is usually understood, tends to view God as an absolute entity existing independently of the world. The theistic God looks more like a being, a ‘self-contained substance’ above and apart from the world, than the pure actuality of subsistent being itself. From Thomas’ perspective, this would mean that the independence of God, as over against the world of finite beings, is conceived wrongly. It is as if the character of subsistence, attributed to a theistically conceived God, is a logical expression by means of which we think of God as separated from the world, as a distinct reality, while Thomas intends to express by subsistence that the being of God is separated through itself from all other beings. The difference is crucial. For Thomas, God is not ‘separated’ from the world as a subsistent entity conceivable apart from his causal relationship to created beings; it is as cause of all beings that God ‘separates’ himself from all his effects by distinguishing those effects from himself. In this sense the ‘concept’ of God is, in truth, the concept of the relationship of God and world, conceived as an ordered plurality of diverse beings, each of which receives its being from the divine source of being. For Thomas there is no way of thinking of God concretely outside the relationship. The independence, or absoluteness, of God characterizes the way He relates as cause to all other things; it is the independence of the perfect goodness of God, who is not under any obligation or necessity to fulfil himself by creating, but who acts out of his own goodness, establishing all other things in being by letting them share in his own perfection.
Rudi te Velde, Aquinas on God, pp. 84-85