Aquinas’ first answer to this question is simply that we cannot know what God is, however, we know what God is not.
The Angelic Doctor begins this section by defining God as not being composed of parts of any kind, therefore being a Trinitarian, not a Unitarian. The three Persons are not three parts of God. Aquinas here is talking about the one nature of God.
The Saint shows God’s simplicity by mentioning nine possible kinds of composition and proves that each one of them cannot by God. Validating that God is not composed of: material parts, matter and form, subjects and nature, substance and attributes, essence and existence, genus and difference, substance and accident, any other composition at all, or composition with other beings.
Next, Aquinas show’s God’s perfection. He deduces that God is supremely actual, and does not have any potential. St Thomas also says that “the perfections of all things are in God…” This means that whatever is perfect in earthly beauty, goodness, intelligence, power or anything at all, is in God, but without their limits. This implies that whatever good, beauty and perfection we love in creatures is found in God more perfectly.
God’s goodness in next to be considered. Aquinas says that all being is good (also known as cosmic optimism). He proves it by saying “the essence of goodness consists in this, that it is in some way desirable. Hence the Philosopher [Aristotle] says ‘Goodness is what all desire.’ Now it is clear that a thing is desirable only insofar as it is perfect… but everything is perfect so far as it is actual. Therefore it is clear that a thing is perfect so far as it actually exists, for it is existence that makes all things actual… hence it is clear that goodness and being are the same really”. In simpler terms: all that is, is good. Another way of grasping the concept is by simply arguing everything that exist is either the Creator or a creature, and the Creator is perfectly good, and any creating of God, anything created by perfect goodness, must also be good. This does not mean that nothing is evil, it means that no being is evil. Nothing is ontologically evil, although some human acts and some human choices are morally evil. Aquinas agrees with Aristotle’s claim that there are only three kinds of goodness: the pleasant, the useful, and the virtuous. All created things are pleasant somehow and useful somehow, but only moral character and moral choices can be virtuous or vicious.
Aquinas then proves that infinity is a divine attribute. There is a major distinction that has to be made between Jewish and Christian philosophy from ancient Greek and Roman philosophy. The pagans considered infinity as an imperfection simply because they thought in material terms. They thought that infinite height or infinite weight was not a perfection. “A perfect work of art has to have limits” (pp 37). But infinite goodness or infinite wisdom are not material attributes, but spiritual attributes, and infinity makes them perfect, not imperfect.
God is then proven to be omnipresent. Aquinas writes “God is in all things not as a part of their essence nor as an accident but as an agent is present to that upon which it works… And since God is being itself by His own essence, created being must be His proper effect, as to ignite the proper effect of fire. Now God causes this effect in things not only when they first being to be, but as long as they are preserved in being, as light is caused in the air by the sun as long as the air remains illuminated. Therefore as long as a thing has being, God must present to it. But being is innermost in each thing and most fundamentally inherent in all things. Hence God is in all things, and inner mostly.” Therefore is being (or existence) is an actuality in each thing, and if God is existence by His very essence, then God is actually, really, literally present in every thing. Now people might say how can God be transcendent (existing apart from and not subject to the limitations of the material universe) and omnipresent? The only way God can be omnipresent is if He was transcendent. “God is infinite existence, transcending any finite essence, and that’s why He can be intimately present to every finite essence, as light can be present to every color only being it transcends all colors [sic]” (pp 38)
Immutability, the attribute that states that God does not change over time, is shown next. The premise is that God is pure actuality, without potentiality. Every time something changes, it begins with something potential; and actualises it. If there is no potential in God, there is no change in God. Aquinas then goes on to show that God is unchangeable throughout time, but eternal (or timeless). Boethius’ definition of eternity is “the simultaneously whole and perfect possession of endless life”. Therefore eternity has no past or future, only present. “Our lives are mostly past and future, and therefore not fully real, because what’s past is dead and no longer real and what’s future is unborn and not yet real. But in God everything is real, including what to usi s past and future.” (pp 38)
Last of all, Aquinas show’s God’s unity. This is by stating that “The unity of God is proved from the infinity of His perfection.” That due to God being infinite in essence, he would have to be one. If there were many gods and the all differed from each other, something would belong to one god that would not belong to the other. If that were true, then one of the gods would not be absolutely perfect, thus voiding them of being a god. Aquinas also proves God’s unity by observing the universe and it’s unity. “[The unity of God] is also shown from the unity of the world. For all things that exist are seen to be ordered to each other [In other words, the universe is like a system] But things that are diverse do not harmonize in the same order unless they are ordered thereto by one.” Aquinas then adds a statement about God’s unity, that it is theological rather than philosophical. In other words, it is a statement of faith, rather than a proof” (pp 39).