Q. 2 “The Existence of God”
Theology, then, is the knowledge of God and of everything else insofar as it is ordered towards him (and that includes everything: to exist for St. Thomas simply is to be ordered towards God). The first main “treatise” of the Summa, accordingly, contains St. Thomas’s doctrine of God. Perhaps the first thing we might observe about Thomas’s doctrine of God is that it is very rational or philosophical. Not until Thomas reaches his discussion of the Trinity in question 27 does Thomas believe that his theology for the first time becomes overtly or formally indebted to revelation (except, as we have seen, insofar as he believes revelation helps keep one’s otherwise purely philosophical understanding of God from falling into error). What this means is that God’s existence and attributes such as the divine simplicity, immutability, omniscience, omnipotence, infinitude, and so forth, are all provable by (an admittedly theologically purged) reason, according to St. Thomas.
But we must begin at the beginning: theology is the knowledge of God, and so the first thing in the order of our knowledge of God is the knowledge of his existence. How do we know God is even there? But note: Thomas’s purpose in this question is not to convince the atheist per se. His goal, rather, is to analyze and arrange according to its proper order our knowledge of God, and our knowledge of God’s existence is at least logically prior to our knowledge of any of his attributes.