St. Thomas, however, did more than integrate Aristotelian metaphysics into a comprehensive theological vision of truth. He gave Aristotelianism a new orientation, a deeper significance. For he distinguished, as Aristotle had not, between the essence, the nature of an object, and its existence. The latter he conceived dynamically as the act which imparts being to essence, which of itself is but the possibility of existing. Existence actualizes this possibility within the limits imposed by the distinctive nature of the essence thus made actual. “This distinction”, writes Professor Knowles, “between essence and existence is vital, it is the shibboleth of Thomism.”
In God, however, there is no distinction between essence and existence. For God’s essence, his nature, is precisely to exist; his nature is existence in its plenitude. “God”, writes Gilson, “is the Being whose whole nature it is to be such an existential act: this is the reason why his most proper name is, He is.”
This metaphysical existentialism, as Gilson has emphasized, derives its origin not from any pagan philosophy, concerned rather with essence, but from the Christian understanding of God’s words to Moses: “I am that I am”: I am the self-subsistent Being, Existence pure and simple, and therefore the source of all created existence.
It was in fact this distinctively Jewish-Christian existentialism which enabled St. Thomas to complete Aristotle’s philosophy of essence and impart it to a novel dynamism. It was not, however, that he added a revealed doctrine to the insights of philosophic speculation. Rather his Christian theology, his Christian experience, enabled his intelligence to perceive a metaphysical truth, to advance further along Aristotle’s road than the philosopher himself. A revealed truth was for St. Thomas a lens focusing his philosophic vision on a metaphysical truth otherwise unseen.
[From The Formation of Christendom by Christopher Dawson, published by Ignatius Press, 2008]