Pantheism is the belief that the universe (or multiverse or whatever is all that exists) is identical with God. It is an ancient and current belief. See inter alia Star Wars or attend any yoga class. Atheists speak like pantheists (see discussions about “spontaneous” effects, creation from “nothing”, etc.).
Chapter 26: That God Is Not The Formal Being Of All Things
1 FROM the foregoing we are able to refute the error of some who have asserted that God is nothing else than the formal being of everything.
2 For this being is divided into substantial and accidental being. Now the divine being is neither the being of a substance nor the being of an accident, as shown above. Therefore it is impossible for God to be the being whereby everything is formally.i
3 Again. Things are not distinct from one another in that they have being, since in this they all agree. If, then, things differ from one another, it follows that either being itself is specified by certain differences added thereto, so that different things have a specifically different being, or that things differ in that being itself is attached to specifically different natures. But the former of these is impossible, because an addition cannot be attached to being in the same way as a difference is added to a genus, as already stated. It remains, therefore, that things differ because they have different natures, to which being is attached in different ways. Now the divine being is not attached to another nature, but is the nature itself, as shown above. If, therefore, the divine being were the formal being of all things, it would follow that all things are simply one…ii
5 Further. That which is common to many is not something besides those many except only logically: thus animal is not something besides Socrates and Plato and other animals except as considered by the mind, which apprehends the form of animal as divested of all that specifies, and individualizes it: for man is that which is truly an animal, else it would follow that in Socrates and Plato there are several animals, namely animal in general, man in general, and Plato himself.iii Much less therefore being itself in general is something apart from all things that have being; except only as apprehended by the mind. If therefore God is being in general, He will not be an individual thing except only as apprehended in the mind. Now it has been shown above that God is something not merely in the intellect, but in reality. Therefore God is not the common being of all.iv
6 Again. Generation is essentially the way to being, and corruption the way to not-being. For the term of generation is the form, and that of corruption privation, for no other reason than because the form makes a thing to be, and corruption makes a thing not to be, for supposing a certain form not to give being, that which received that form would not be said to be generated. If, then, God were the formal being of all things it would follow that He is the term of generation. Which is false, since He is eternal, as we have shown above.v
7 Moreover. It would follow that the being of every thing has been from eternity: wherefore there would be neither generation nor corruption. For if there were, it would follow that a thing acquires anew a being already pre-existing. Either then it is acquired by something already existing, or else by something nowise pre-existing. In the first case, since according to the above supposition all existing things have the same being, it would follow that the thing which is said to be generated, receives not a new being but a new mode of being, and therefore is not generated but altered. If on the other hand the thing nowise existed before, it would follow that it is made out of nothing, and this is contrary to the essence of generation. Consequently this supposition would wholly do away with generation and corruption: and therefore it is clear that it is impossible…vi
We skip the next six arguments, which refute an error not of main interest to us.
———————————————————————————— Sum. Th. P. I., Q. iii., A. 8.
 Ch. xxv.
 Ch. xxv.
 Ch. xxii.
 Ch. xv.
 Ch. xiii.
 Ch. xv.
i“This being” is the pantheistic deity, if it existed. Obviously, the universe is made of parts, is in potential, and all those things we already know God cannot be. This is probably the simplest proof in the whole book! So obvious is this that we’ll skip around the remaining arguments, though there is plenty there that is of interest.
iiThere was some confusion about this in the past. If you exist and I exist (and we do) then we both share existence, or being. But after that, we begin to differ. That’s all this means, and Aquinas draws the implication in the next sentence. There are not different kinds of to exist. The takeaway point is that in God existence is essence, or nature.
iiiThat is, we can know the essence of animal, and other essences, too! Once you grasp this seemingly simple point, boy howdy do things change.
ivIn other words, God cannot be a thing which only exists in your imagination.
vA good review: things which are in existence, have being, have form and matter. Take away the form of you and what is left? Nothing but dust. The form of man is his soul.
viI find this argument beautiful. Put another way around, if the universe were God, then nothing could change; things change; therefore the universe is not God. Being cannot alter into new ways of being Being, and nothing can come from nothing. If God were there universe, it would be a dull constant unchanging void with not even a seething quantum “vacuum” to liven it up.