He's making some positive statements on the essence of God.
"Book I:Chapter 22: That in God being and essence are the same.
This must be understood in light of the fact that Aquinas constantly nails home the point that the rational creature of his own power cannot see the essence of God. Hence, for him to say that God's essence is his existence is not to say that he knows the essence of God. He is merely denying (i.e. using the apophatic approach) that God suffers from the same division of essence and existence that we do. Or in other words, he saying that God does not depend on any other being. Ask Aquinas what it is like for a being to be such that essence and existence coincide, and he would say, "I dunno." In fact, Aquinas believes that this unity of essence and existence is the protection of God's divine transcendence:
"It follows, therefore, that to know self subsistent being (a being in which essence and existenec coincide) is natural to the divine intellect alone; and this is beyond the natural power of any created intellect; for no creature is its own existence, forasmuch as its existenec is participated." ST. I. Q. 12. A. 4
From what was proved above, however, we can further prove that His essence or quiddity is not something other than His being [...] If, then, the divine essence is something other than its being, the essence and the being are thereby related as potency and act. But we have shown that in God there is no potency, but that He is pure act. God's essence, therefore, is not something other than His being. "
Again, when reading the above, we must keep in mind statements from Aquinas like the following: "But we cannot know in what God's essence consists, but solyt in what it does not consist." S.T. I. Q2. a. 3 Thus, wehn he talks about God's essence and exsitence coinciding, or states that God is pure act, he is merely denying (again here is the apophatic approach)... He is deying that God is composed of essence and existence, or act and potency like we are. Aquinas has no idea what it is like to lack such compositions. He merely knows that he can deny those compositions of God.
The ST (1:12:1) has something about the essence of God too:
"Since everything is knowable according as it is actual, God, Who is pure act without any admixture of potentiality, is in Himself supremely knowable. But what is supremely knowable in itself, may not be knowable to a particular intellect, on account of the excess of the intelligible object above the intellect; as, for example, the sun, which is supremely visible, cannot be seen by the bat by reason of its excess of light.
Therefore some who considered this, held that no created intellect can see the essence of God. This opinion, however, is not tenable. For as the ultimate beatitude of man consists in the use of his highest function, which is the operation of his intellect; if we suppose that the created intellect could never see God, it would either never attain to beatitude, or its beatitude would consist in something else beside God; which is opposed to faith. For the ultimate perfection of the rational creature is to be found in that which is the principle of its being; since a thing is perfect so far as it attains to its principle. Further the same opinion is also against reason. For there resides in every man a natural desire to know the cause of any effect which he sees; and thence arises wonder in men. But if the intellect of the rational creature could not reach so far as to the first cause of things, the natural desire would remain void.
Hence it must be absolutely granted that the blessed see the essence of God. "
[/quote] Now here is a true point of contention between Catholics and Eastern Orthodox. Catholics and Orthodox Christians both agree that no one can comprehend the essence of God. Even by Divine aide, we will never be able to do so. But Catholics believe that is possible to apprehend (experience) the essence of God without comprehension. This distinction protects us from falling into pantheism. What is more, even this apprehension (in heaven) without comprehension is only possible by participation in the uncreated grace of God. Eastern Orthodox Christians, on the other hand say that there is not even an apprehension of God's essence, but that creatures only participate in his Divine energies. In either case, I think both are saying that God is absolutely transcendent, but that we do become divinized. Catholics say it by arguing that we don't comprehend God's essence, but we apprehend it by aide of God's uncreated grace (in heaven). Eastern Orthodox Christians say it by saying that we have no experience of God's essence at all, not even by apprehension by participation in God's grace, but we do participate in God's life by means of his energies. In either case, I'm a philosopher and not a theologian, so I will not delve into which language is best used here. I will say that if anyone thinks we know what God's essence is in itself, that person is a heretic. I would also say that anyone who denies theosis is also a heretic. I think Aquinas would agree with me.
Then the SCG 1:48:1 has:
"Now, it appears from what we have said that primarily. and essentially God knows only Himself. "
Quite confusing all this.
Only God knows himself, we do not.